Level shifting and level-shifting amplifier circuits

Abstract

Level shifting and amplified level shifting circuit topologies are provided that include two or more level shifting or amplified level shifting circuits. The level shifting circuits receive a variable and fixed input and generate a variable and fixed output that are level shifted with respect to the input signals. The amplified level shifting circuits receive a variable and fixed input and generate a variable and fixed output that are level shifted and amplified with respect to the input signals. These circuits may be utilized to form a detection circuit that detects a difference in the output signals.

Description

FIELD OF THE INVENTION

The present invention relates to level shifting and/or amplifying level shifting circuitry. Particular utility for the present invention includes a detector circuit that can be used as a wakeup circuit to generate a wakeup signal that may be used in power management systems; although the present invention is broadly applicable to a level shifting and/or level shifting and amplifying circuit topologies that may be implemented in a broad range of applications.

DESCRIPTION OF RELATED ART

FIG. 1 depicts a generalized block diagram for a bipolar signal detector 10 known in the art. The detector includes a first comparator 12 that compares a sensing signal V_sen with a positive threshold voltage V_H, and a second comparator 14 that compares the sensing signal V_sen with a negative threshold signal V_L. The sensing signal V_sen can be derived from an associated system (not shown), for example, a sense resistor. Gate 16 generates an output signal V_det based on the outputs of one or both of the comparators. As shown by the polarity of the comparators, if V_sen is less than V_L or greater than V_H, then V_det is HIGH (or LOW) indicating that V_sen is greater than (absolute value) the threshold voltages. One disadvantage of this topology is that a negative power supply is required for the comparators and for the negative threshold voltage V_L. Also, if the sensing signal is small compared to the comparator offset, the comparator offset may generate erroneous outputs. In other words, if the difference between V_H and V_L is within the offset of the comparator, the circuit 10 becomes incapable of detecting a small sensed signal within this range.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a level shifting circuit topology. One exemplary topology includes a first and second level shifting circuits wherein said second level shifting circuit generating a level shifted output signal from a variable input signal, the said first level shifting circuit generating a fixed level shifted output threshold signal from a fixed input reference signal. The level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

Another exemplary level shifting circuit is provided that includes a first, second and third level shifting. The second level shifting circuit generates a level shifted output signal from a variable input signal. The first and third level shifting circuits generate fixed, level shifted output threshold signals from a fixed input reference signal. The level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

In another aspect, the present invention provides an amplifying level shifting circuit topology. One exemplary topology includes a first and second amplifying level shifting circuits. The second amplifying level shifting circuit generates an amplified and level shifted output signal from a variable input signal. The first amplifying level shifting circuit generates a fixed, amplified and level shifted output threshold signal from a fixed input reference signal. The amplified level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

Another exemplary amplifying level shifting circuit topology is provided that includes comprising a first, second and third amplifying level shifting circuits. The second amplifying level shifting circuit generates an amplified and level shifted output signal from a variable input signal. The first and third amplifying level shifting circuits generate fixed, amplified and level shifted output threshold signals from a fixed input reference signal. The amplified level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

In a specific, exemplary circuit implementation, the present invention provides a detection circuit that includes a first and second level shifting circuits and a comparator. The second level shifting circuit generates a level shifted output signal from a variable input signal, and the first level shifting circuit generates a fixed level shifted output threshold signal from a fixed input reference signal. The comparator receives the level shifted output signal and the fixed level shifted output signal and generates a signal indicative of the difference between the level shifted output signal and the fixed level shifted output signal.

In another specific, exemplary circuit implementation, the present invention provides a detection circuit that includes a first and second amplifying level shifting circuits and a comparator. The second amplifying level shifting circuit generates an amplified and level shifted output signal from a variable input signal, and the first amplifying level shifting circuit generating a fixed, amplified and level shifted output threshold signal from a fixed input reference signal. The comparator receives the amplified level shifted output signal and the fixed, amplified level shifted output signal and generating a signal indicative of the difference between the amplified level shifted output signal and the fixed, amplified level shifted output signal.

It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments and methods of use, the present invention is not intended to be limited to these preferred embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims.

Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a generalized block diagram for a conventional bipolar signal detector;

FIG. 2A depicts one exemplary circuit diagram of a level shifting circuit topology according to the present invention;

FIG. 2B depicts an exemplary individual level shifting circuit as used in the topology of FIG. 2A;

FIG. 2C depicts a graph of the DC transfer curves of the topology of FIG. 2A;

FIG. 3 depicts another exemplary circuit diagram of a level shifting circuit topology according to the present invention;

FIG. 4A depicts an exemplary circuit diagram of a level-shifting and amplifying circuit topology according to the present invention;

FIG. 4B depicts an exemplary individual level-shifting and amplifier circuit as used in the topology of FIG. 4A;

FIG. 4C depicts the DC transfer curves for the circuit of the topology of FIG. 4A; and

FIG. 5 depicts another exemplary circuit diagram of a level-shifting and amplifying circuit topology according to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Level Shifting Circuit Topologies

FIG. 2A depicts one exemplary circuit diagram of a level shifting circuit topology according to the present invention. The following description of this topology is directed to a level shifting the input signals when V_sen is close to power supply VSS. As opposed to the the topology of FIG. 1, the present invention provides two or more level shifting circuits, and in the exemplary embodiment three level shifting circuits B₁, B₂ and B₃ are provided, as described below.

Referring briefly to FIG. 2B, which is a separate schematic for level shifter B₁˜B₃ in FIG. 2A. Each level shifter consists of an active load transistor M_P1 (i=1, 2, 3), a resistor R_i (i=1, 2, 3) and a source follower M_F1 (i=1, 2, 3). The input signal V_m goes into the gate of source follower M_F1 and output from its source, then go through the resistor R_i, form the final output signal V_o.

Using small signal model analysis, we get:V_o=[g_mF1/(g_dsF1+g_mF1+g_mbF1)]V_in+I_b{[(R_i+1/(g_dsF1+g_mbF1)]//1/g_dsP1}  (1)Normally, g_mF1>>g_mbFi>>g_dsF1, (R_i+1(g_dsF1+g_mFi+g_mbFi)) <<1/g_dsP1So, equation (1) can be simplified as:V_o≈V_in+I_bR_i+I_b(1/g_mFi)(i=1, 2, 3)  (2)

From equation (2), the level shifter has a gain approximately equal to one, i.e., the level shifter has little or no amplification function. But the level shifter does have a shifting aspect represented by I_b(R₁+1/g_mF1). Thus, as will be apparent to one skilled in the art, by selecting values of R_i and g_mF1 one can select a desired shifting value. Shifting, in this case, refers to amplitude shifting. In the circuit of FIG. 2A, and assuming that V_sen is a signal that is close to a supply voltage VSS, it may be desirable to shift the input signals (V_ref and V_sen, more fully described below) toward VDD. Although not a requirement of the present invention, shifting the amplitude of the input signals in this manner may be useful so that external circuitry (e.g., the comparators 12 and 14) operates in the linear region of operation for those devices.

In the exemplary embodiment of FIG. 2A, to simplify the analysis and construction, the three level shifters B₁, B₂ and B₃ have approximately the same dimension load transistor M_P1, M_P2 and M_P3. Also, the level shifters have approximately the same dimension source follower M_F1, M_F2 and M_F3: Of course, the term approximately is to be construed broadly, and may depend on the particular tolerance required or the particular operating environment. Additionally, it is not a requirement of the present invention to have approximately the same load dimension for the transistors, but rather, is described herein as only one exemplary construction.

Shifting the input signals is therefore a function of the resistors R₁, R₂, and R₃ as shown in equation (2). In this exemplary embodiment, the resistor R₂ associated with the sensed input signal V_sen may be chosen to be shifted so that it resides between two threshold signals, i.e., R₁<R₂<R₃. So according to equation (2), each level shifter has a different shifting value.

As shown in FIG. 2C, because the gain is roughly equal one, the DC transfer curves are three parallel 45 degree straight line for each level shifter B₁, B₂ and B₃ The DC transfer curves represent the input/output of each of the three level shifting circuits. Level shifter B₂ will shift the signal to be detected V_sen to V_o′, which may be optimized to around the middle of the power supply, e.g., (VDD−VSS)/2. Level shifter B₁ generates a lower threshold V_L′, and the input to B₁ is a reference signal V_ref. Level shifter B₃ generates an upper threshold V_H from the reference signal V_ref. Thus, level shifters B₁ and B₃ generate a fixed, level shifted output from a fixed input, where the output is level shifted according to equation (2).

As mentioned above, the exemplary topology of FIG. 2A assumes that V_sen is close to or equal to the power supply VSS. “Close to” is to be construed broadly and may depend on, for example, the tolerances required for a given application. For example, “close to” may be defined as within 10% of a power supply voltage. Accordingly, V_ref is selected to be close to or equal to VSS.

For equation (2), the threshold|V_H′−V_O′|=I_b(R₃−R₂)  (3a)|V_O′−V_L′|=I_b(R₂−R₁)  (3b)Because the gain of the level shifter is roughly one, the original threshold|V_H−V_ref|=|V_H′−V_O′|=I_b(R₃−R₂)  (4a)|V_ref−V_L|=|V_O′−V_L′|=I_b(R₂−R₁)  (4b)

As we can see from FIG. 2C, when the signal to be detected V_sen is between V_L and V_H then the shifted signal V_o′ will be between V_L′ and V_H′, otherwise V_o′ will be greater than V_H′ or less than V_L′. When the topology of FIG. 2A is used as a detection circuit (as described with reference to FIG. 1) using comparators 12 and 14 and XOR gate 16, then when V_sen is between V_L and V_H, V_det will keep the same state, either high or low. But when V_sen<V_L or V_sen>V_H, then V_o′<V_L′ or V_o′>V_H′ and V_det will change state, either from high to low or from low to high.

In FIG. 2A, the bias circuit generates the bias current I_b.I_b=Vt ln(A)/R_o  (5)

Where A is the area ratio of transistor Q₁, and Q₂. Substrate (5) into (4),|V_H−V_ref|=Vt ln(A)(R₃−R₂)/R_o  (6a)|V_ref−V_L|=Vt ln(A)(R₂−R₁)/R_o  (6b)

From (6a) and (6b), the threshold is determined by transistor area ratio and resistor ratio and resistor differences, which can be accurately controlled in IC process. Of course, for those skilled in the art, the bias circuit is not necessarily the same as in FIG. 2A. Other kinds of circuits can be used to generate the bias current I_b.

Also, depending on the desired application, it may not be necessary to have both the lower threshold V_L and upper threshold V_H Instead, it may be desirable that only one threshold, either V_L or V_H is needed, in this case, one level shifter and one comparator, either level shifter B₁ and comparator 12 or level shifter B₃ and comparator 14 can be omitted.

FIG. 3 is another exemplary level shifting topology of the present invention, and may be used, for example, when V_sen is very close to power supply VDD. For those skilled in the art, this exemplary embodiment follows the same principles as described above with reference to FIGS. 2A, 2B and 2C. The changes are the source followers M_F1˜M_F3 change from PMOS to NMOS, and the active load change from PMOS M_P1˜M_P3 to NMOS M_N1˜M_N3. The operation of FIG. 3 is similar to FIG. 2A except that the outputs of the level shifting circuits are shifted away from VDD and towards VSS. Of course, the circuits of FIGS. 2A and 3 are equally applicable for bipolar signal level shifting, i.e., where V_sen resides at +/−0 Volts. In such a case, it may be desirable to down shift (FIG. 3) or up shift (FIG. 2A) the outputs so that the output reference signals V_H′ and V_L′ are both positive or both negative. These and other modifications may all be obtained using the topology of FIGS. 2A and/or 3.

As previously stated, the exemplary embodiments of FIG. 2A and FIG. 3 may be adapted to shift the signal to be detected, V_sen, to be around the middle of the power supply. If the topologies of FIG. 2A and/or FIG. 3 are adapted to include comparators 12 and 14, by shifting to around the middle of the power supply region allows the comparators to work in their linear region.

But if the signal to be detected, V_sen is small, e.g. the threshold |V_H−V_sen| or |V_sen−V_L| is comparable to the offset of comparator 12 and/or 14, then it may be desirable to amplify the level shifted signals. Amplified level shifting circuits are described below.

Amplified Level Shifting Circuit Topologies

FIG. 4A is an exemplary amplified level shifting circuit topology. Like previous exemplary embodiments, the circuit of FIG. 4A assumes V_sen is close to or equal to VSS. In this exemplary embodiment, three pre-amplifier stages A₁, A₂ and A ₃ are used to both amplify and level shift input signals, as will be detailed below.

FIG. 4B is the schematic for pre-amplifier A₁˜A₃ in FIG. 4A. Each pre-amplifier consists of two branches. M_PA1, M_NA1, and R₁(1=1, 2, 3) form the left branch, M_PB1, M_NB1 and R_b form the right branch. The left branch shifts the input signal V_in, which is close to VSS, to an appropriate level V_g,V_g=V_in+I_bR₁+V_gsNA1  (7)

Where (I_bR₁+V_gsNAi) is the shifted factor. In this exemplary embodiment, the right branch is a common source amplifier. Through small signal AC analysis, we get: $\begin{array}{cc}{V}_o=\left\{g_{\mathrm{mNBi}}/\left[R_b{g}_{\mathrm{dsPBi}}\left(g_{\mathrm{mNBi}}+g_{\mathrm{mbNBi}}+g_{\mathrm{dsNBi}}\right)-\left(g_{\mathrm{dsNBi}}+g_{\mathrm{dsPBi}}\right)\right]\right\}{V}_g\approx \left\{g_{\mathrm{mNBi}}/\left[R_b{g}_{\mathrm{mNBi}}{g}_{\mathrm{dsPBi}}-\left(g_{\mathrm{dsNBi}}+g_{\mathrm{dsPBi}}\right)\right]\right\}{V}_g& \left(8\right)\\ \approx \beta V_g& \left(9\right)\end{array}$

From (8), we see that the right branch has a gain β. Thus, one skilled in the art will recognize that by selecting an appropriate resistor R_b value, the pre-amplifier will work in the linear region having a gain β. Thus, a small difference in input signal V_g, which is the shifted signal of V_in, will generate a larger difference in output V_o.

Referring again to FIG. 4A, the three pre-amplifiers A₁, A₂, and A₃ have approximately the same dimension for all the relative transistors and approximatly the same value for R_b, but, in order to shift the input signals a different value is selected for each of R₁, R₂ and R₃, To shift the signal to be detected V_sen to a point between two threshold signals, then the values are selected as R₁<R₂<R₃. According to equation (7) and (8), each preamplifier circuit shifts the input signal and thus generates a different transfer curve, as shown in FIG. 4C.

Pre-amplifier A₂ shifts and amplifies the signal to be detected V_sen to a desired output level V_o″. As before, this output value may be selected to be around the middle of the power supply. Pre-amplifier A₁ generates an upper threshold V_H″ from a fixed reference signal V_ref. Pre-amplifier A₃ is used to generate the lower threshold V_L″ from the reference signal V_ref. In the example of FIG. 4A, V_ref is selected to be close to or equal to VSS.

According to equation (7), the original threshold|V_H−V_ref|=I_b(R₃−R₂)  (10a)|V_ref−V_L|=I_b(R₂−R₁)  (10b)

But after the pre-amplifier, the original threshold will be amplified to|V_O″−V_L″|=βI_b(R₃−R₂)  (11a)|V_H″−V_O″|=βI_b(R₂−R₁)  (11b)

As we can see from FIG. 4C, when the signal to be detected V_sen is between V_L and V_H then the shifted and amplified signal V_o″ will be between V_L″ and V_H″, otherwise V_o″ will be greater than V_H″ or less than V_L″. When the topology of FIG. 4A is used as a detection circuit (as described with reference to FIG. 1) using comparators 12 and 14 and XOR gate 16, then when V_sen is between V_L and V_H, V_det will keep the same state, either high or low. But when V_sen<V_L or V_sen>V_H, then V_o″<V_L″ or V_o″>V_H″ and V_det will change state, either from high to low or from low to high. So, even if the original threshold is only about 1 mV, which is comparable to the comparator offset, the gain can be adjusted such that the output signals are well larger than the comparator offset. For example, β may be selected to be equal to about 100, then the amplified threshold will be 100 mV, which is typically much larger than the offset of a typical comparator.

Again, the bias circuit in FIG. 4A will generate the bias current.I_b=Vt ln(A)/R_o

Then, the original threshold in (10a) and (10b) will be|V_H−V_ref|=Vt ln(A)(R₃−R₂)/R_o  (12a)|V_ref−V_L|=Vt ln(A)(R₂−R₁)/R_o  (12b)

The threshold is determined by transistor area ratio and resistor ratio and resistor differences, which can be precisely controlled in IC Process. Of course, those skilled in the art will recognize that alternative bias circuits may be used to generate the bias current I_b. Also, depending on the desired application, it may not be necessary to have both the lower and upper threshold V_L and V_H. In this case, one pre-amplifier and/or one comparator, either A₁ and comparator 12 or A₃ and comparator 14 can be omitted.

FIG. 5 is another exemplary amplified level shifting topology of the present invention, and may be used, for example, when V_sen is very close to power supply VDD. For those skilled in the art, this exemplary embodiment follows the same principles as described above with reference to FIGS. 4A, 4B and 4C. The operation of FIG. 5 is similar to FIG. 4A except that the input signals are shifted away from VDD and towards VSS. Of course, the circuits of FIGS. 4A and 5 are equally applicable for bipolar signal level shifting, i.e., where V_sen resides at +/−0 Volts. In such a case, it may be desirable to shift down (FIG. 5) or shift up (FIG. 4A) the outputs so that the output reference signals V_H″ and V_L″ are both positive or both negative. These and other modifications may all be obtained using the topology of FIGS. 4A and/or 5.

Note that if the input signal is a small signal, the topologies of FIGS. 4A and 5 may be used as a detection circuit for “small signal” detection. Small signal may mean, for example, that the signal to be detected is within the tolerance range of given components. Note further that the amplifying function of this circuit topology may be chosen to make the output threshold to be larger than the offset voltage of a comparator (12 or 14). Those skilled in the art will recognize numerous modifications to the exemplary topologies of the present invention. For example, those skilled in the art will recognize that many different level shifting circuits exist which may be modified to be biased with respect to one another so that the outputs are shifted in a manner according to the present invention. Indeed, the specific circuit topologies disclosed herein are only exemplary, and other level shifting topologies may be used instead. Likewise, the exemplary level shifting and amplifying circuit topologies described herein may be replaced with other level shifting and amplifying circuit topologies as are known in the art, without departing from the present invention. All such modifications are deemed within the scope of the present invention, only as limited by the appended claims.

Claims

1. A level shifting circuit topology, comprising first, second, and third level shifting circuits, said first level shifting circuit generating a level shifted output signal from a variable input signal, said second level shifting circuit generating a fixed level shifted output threshold signal from a fixed input reference signal, and said third level shifting circuit generating a second fixed level shifted output threshold signal from said fixed input reference signal; said first, second and third level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other.

2. The level shifting circuit as claimed in claim 1, said second level shifting circuit being biased so that its DC transfer curve is shifted between the shifted DC transfer curves of said first and third level shifting circuits.

3. The level shifting circuit topology as claimed in claim 1, wherein said first, second and third level shifting circuits each comprise: a load transistor coupled to a first voltage source; a source follower transistor coupled to a second voltage source, and

4. The level shifting circuit topology as claimed in claim 3, further comprising a bias circuit generating a bias voltage for biasing said load transistor, said fixed input signal or said variable input signal controlling the conduction state of said source follower transistor.

5. The level shifting circuit topology as claimed in claim 3, wherein a first bias resistor associated with said first level shifting circuit has a value less than a second bias resistor associated with said second level shifting circuit, which has a value less than a third bias resistor associated with said third level shifting circuit.

6. The level shifting circuit topology as claimed in claim 1, wherein said DC transfer curves are right shifted with respect to a supply voltage.

7. The level shifting circuit topology as claimed in claim 1, wherein said DC transfer curves are left shifted with respect to a supply voltage.

8. The level shifting circuit topology as claimed in claim 1, wherein said DC transfer curves are shifted to be approximately in the middle of two supply voltages.

9. An amplifying level shifting circuit topology comprising first and second amplifying level shifting circuits, said first amplifying level shifting circuit generating an amplified and level shifted output signal from a variable input signal, and said second amplifying level shifting circuit generating a fixed, amplified and level shifted output threshold signal from a fixed input reference signal, said first and second amplifying level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other; said amplifying level shifting circuit topology further comprising a third amplifying level shifting circuit generating a second fixed, amplified level shifted output threshold signal from said fixed input reference signal, said first, second and third amplifying level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other.

10. The amplifying level shifting circuit as claimed in claim 9, wherein said second amplifying level shifting circuit being biased so that its DC transfer curve is shifted between the shifted DC transfer curves of said first and third amplifying level shifting circuits.

11. The amplifying level shifting circuit topology as claimed in claim 9, wherein said first, second and third level shifting circuits each comprise: a first branch circuit for level shifting said input signal, said first branch further comprising a load resistor for determining the shift of said input signal, and; a second branch for amplifying said input signal.

12. The amplifying level shifting circuit topology as claimed in claim 11, further comprising a bias circuit generating a bias voltage for biasing said first branch.

13. The amplifying level shifting circuit topology as claimed in claim 11, wherein a first load resistor associated with said first amplifying level shifting circuit has a value less than a second load resistor associated with a second amplifying level shifting circuit, which has a value less than a third load resistor associated with a third amplifying level shifting circuit.

14. A level shifting circuit topology, comprising first, second and third level shifting circuits, said second level shifting circuit generating a level shifted output signal from a variable input signal, and said first and third level shifting circuits generating fixed, level shifted output threshold signals from a fixed input reference signal; said level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other.

15. The level shifting circuit as claimed in claim 14, said second level shifting circuit being biased so that its DC transfer curve is shifted between the shifted DC transfer curves of said first and third level shifting circuits.

16. An amplified level shifting circuit topology, comprising first, second and third amplifying level shifting circuits wherein said second amplifying level shifting circuit generates an amplified and level shifted output signal from a variable input signal, and said first and third amplifying level shifting circuit generate fixed, amplified and level shifted output threshold signals from a fixed input reference signal; said amplifying level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other.

17. The amplified level shifting circuit as claimed in claim 16, said second amplifying level shifting circuit being biased so that its DC transfer curve is shifted between the shifted DC transfer curves of said first and third amplifying level shifting circuits.

18. A detection circuit, comprising: first, second and third level shifting circuits, said first level shifting circuit generating a level shifted output signal from a variable input signal, said second level shifting circuit generating a fixed level shifted output threshold signal from a fixed input reference signal, and said third level shifting circuit generating a second fixed level shifted output signal; a first comparator receiving said level shifted output signal and said fixed level shifted output signal and generating a signal indicative of the difference between said level shifted output signal and said fixed level shifted output signal; and a second comparator receiving said level shifted output signal and said second fixed level shifted output signal and generating a second signal indicative of the difference between said level shifted output signal and said second fixed level shifted output signal.

19. The detection circuit as claimed in claim 18, further comprising a logic gate receiving said first and second signals and generating a detection signal having a state defined by said first or second signal.

20. The detection circuit as claimed in claim 19, said logic gate comprising an XOR gate.

21. The detection circuit as claimed in claim 18, said level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other.

22. The detection circuit, as claimed in claim 18, said level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other.

23. The detection circuit as claimed in claim 22, said second level shifting circuit being biased so that its DC transfer curve is shifted between the shifted DC transfer curves of said first and third level shifting circuits.

24. A detection circuit comprising: first and second amplifying level shifting circuits, said first amplifying level shifting circuit generating an amplified and level shifted output signal from a variable input signal, and said second amplifying level shifting circuit generating a fixed, amplified, and level shifted output threshold signal from a fixed input reference signal; a comparator receiving said amplified and level shifted output signal and said fixed, amplified and level shifted output signal and generating a signal indicative of the difference between said amplified and level shifted output signal and said fixed, amplified, and level shifted output signal; a third amplifying level shifting circuit generating a second fixed, amplified level shifted output signal; and a second comparator receiving said amplified and level shifted output signal and said second fixed, amplified and level shifted output signal and generating a second signal indicative of the difference between said amplified and level shifted output signal and said second fixed, amplified and level shifted output signal.

25. The detection circuit as claimed in claim 24, further comprising a logic gate receiving said first and second signals and generating a detection signal having a state defined by said first or second signal.

26. The detection circuit as claimed in claim 25, wherein said logic gate comprises an XOR gate.

27. The detection circuit as claimed in claim 24, said amplifying level shifting circuits being biased with respect to each other so that each has a DC transfer curve that is shifted with respect to the other.

28. The detection circuit as claimed in claim 27, said second amplifying level shifting circuit being biased so that its DC transfer curve is shifted between the shifted DC transfer curves of said first and third amplifying level shifting circuits.