Circuit for biasing a transistor and related system and method

Abstract

An embodiment of a circuit for biasing a transistor such as an amplifier transistor includes reference and bias nodes, and includes buffer, reference, and feedback stages. The reference node receives a reference current, and the bias node, which is for coupling to the transistor, carries a bias signal. The buffer stage buffers the reference node from the bias node. The reference stage generates the bias signal from the reference current, and the bias signal causes the transistor to conduct a bias current that is proportional to the reference current. And the feedback stage is coupled between the reference and bias nodes. As compared to known bias circuits, such a bias circuit may reduce the amplitude and duration of a transient overshoot in the bias current of a field-effect transistor when the DC component of the transistor's drain voltage transitions from one value to another value. Such a bias circuit may also reduce the difference between the values of the bias current through the transistor for different supply voltages. And such a bias circuit may reduce the difference between the predicted and actual values of the bias current through the transistor for a given input voltage such as that between the gate and the source of a field-effect transistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional RF amplifier stage.

FIG. 2 is a diagram of the DC bias circuit of FIG. 1.

FIG. 3 is a plot of V_transmission of FIG. 1 versus time during a high-to-low transition of V_transmission.

FIG. 4 is a plot of I_bias of FIG. 1 versus time in response to the high-to-low transition of V_transmission (FIG. 3).

FIG. 5 is a plot of the magnitude of the actual scale factor s_actual versus the magnitude of I_ref for the RF amplifier stage of FIGS. 1 and 2.

FIG. 6 is a diagram of an embodiment of an RF amplifier stage.

FIG. 7 is a plot of I_bias from FIG. 6 versus time in response to a high-to-low transition of V_transmission (FIG. 3).

FIG. 8 is a plot of V_bias from FIG. 6 versus time in response to a high-to-low transition of V_transmission (FIG. 3).

FIG. 9 is a plot of the magnitude of the actual scale factor s_actual versus the magnitude of I_ref for the RF amplifier stage of FIG. 6.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in the art to make and use one or more embodiments of the invention. The general principles described herein may be applied to embodiments and applications other than those detailed below without departing from the spirit and scope of the invention. Therefore the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein.

FIG. 6 is a diagram of an embodiment of an amplifier stage 50, where like numerals reference components common to this amplifier stage and to the amplifier stage 10 of FIG. 1. For clarity, the RF signal generator 16, DC blocking capacitor 22, and choke 18 of FIG. 1 have been omitted from FIG. 6.

The amplifier stage 50 is similar to the amplifier stage 10 of FIG. 1, except that the stage 50 includes a modified DC bias circuit 52.

As compared to the DC bias circuit 14 (FIG. 2) of the amplifier stage 10, the bias circuit 52 may, as described below, reduce the magnitude and duration of the transient overshoot in I_bias caused by a transition of V_transmission. The bias circuit 52 may also reduce the difference between the quiescent values of I_bias for different values of V_transmission, and may reduce the difference between the value of I_bias predicted by equations (1) and (2) and the actual value of I_bias.

Still referring to FIG. 6, in addition to the current source 30, the reference stage 34, and the buffer stage 36, the DC bias circuit 52 includes a feedback stage 54, which includes a sense transistor 56 and feedback resistors 58, 60, and 62. The response, with proper scaling, of the sense transistor 56 is matched to the amplifier transistor 12, although the W/L ratio of the sense transistor 56 may be smaller than that of the transistor 12 so that the sense transistor draws a quiescent current I_sense that is proportionally smaller than I_bias.

Assume that an embodiment of the amplifier stage 50 has the transistor parameters, resistor values, and supply-voltage values as respectively shown in the following Tables IV-VI.

	TABLE IV
	Amplifier	Reference	Buffer	Sense
	Transistor 12	Transistor 42	Transistor 46	Transistor 56
Channel	960 μm	100 μm	100 μm	100 μm
Width
(assuming all
transistors
have the
same channel
length)

	TABLE V
				Resistor	Resistor
	Resistor 40	Resistor 44	Resistor 58	60	62
Resistance	7.12 KΩ	1.39 KΩ	1.39 KΩ	100 Ω	5.0 KΩ

	TABLE VI
	Vref	Vbuffer
	Voltage	1.3 V	0.9 v

According to a computer analytical simulation of this embodiment of the amplifier stage 50, the quiescent currents that flow in the amplifier stage for V_transmission=1.0 V and 3.6 V are shown in Table VII.

	TABLE VII
		Current @	Current @
	Component	Vtransmission = 1.0 V	Vtransmission = 3.6 V
	Iref	0.1	mA	0.1	mA
	Ibias	1.09	mA	1.33	mA
	Ibuffer	0.22	mA	0.21	mA
	I58	0.22 mA + 0.12 μA	0.22	mA
	Ifeedback	0.12	μA	3.34	μA
	Isense	0.22	mA	0.22	mA

The operation of an embodiment of the amplifier stage 50 having the transistor parameters, resistor values, supply-voltage values, and quiescent currents shown in Tables IV-VII is now discussed.

FIG. 7 is a plot of I_bias of FIG. 6 versus time in response to the high-to-low transition of V_transmission shown in FIG. 3.

As shown in FIG. 7 and as discussed below, the DC bias circuit 52 reduces the amplitude and duration of the negative transient overshoot experienced by I_bias when V_transmission transitions from V_high to V_low as compared to the transient overshoot experienced by I_bias of the amplifier stage 10 (FIG. 1) operating under similar conditions. In one embodiment, the bias circuit 52 reduces the amplitude of the negative overshoot by over 50%.

Because the sense transistor 56 is matched to the amplifier transistor 12 and is powered by the same supply voltage V_transmission, I_sense also experiences a negative transient overshoot in response to the high-to-low transition of V_transmission.

The negative overshoot of I_sense reduces the voltage across the resistor 58, and thus also reduces the voltage at the gate G of the reference transistor 42 via the feedback transistor 60.

The reduced voltage at the gate G of the reference transistor 42 reduces I_ref, and thus increases the voltage at the drain D of the reference transistor.

As shown in FIG. 8, which is a plot of V_bias versus time, in response to the transient overshoot of I_bias shown in FIG. 7, the buffer transistor 46, which is configured as a source follower, couples the voltage increase at the drain D of the reference transistor 42 to the bias node 38, and thus increases V_bias (note that FIGS. 7 and 8 have different time scales).

This increase in V_bias causes an increase in I_bias, and this increase in I_bias opposes, and thus reduces the amplitude and duration of, the negative transient overshoot in I_bias caused by the high-to-low transition of V_transmission. One or more parameters of the DC bias circuit 52 may be adjusted to set the amounts by which the DC bias circuit reduces the amplitude and duration of the negative transient overshoot in I_bias.

Thereafter, I_bias and the other currents settle to the quiescent values shown in the second column of Table VII, and V_bias settles to a quiescent value equal to the product of I_buffer and the value of the resistor 44.

In a similar manner, the DC bias circuit 52 may reduce the amplitude and duration of the positive transient overshoot in I_bias when V_transmission transitions from V_low to V_high as compared to the amplitude and duration of the positive transient overshoot in I_bias of the amplifier stage 10 (FIG. 1).

Referring again to FIG. 6, using the same feedback action as described above, the DC bias circuit 52 may also reduce the difference between the quiescent values of I_bias at V_transmission=V_high and V_transmission=V_low as compared to the difference between the corresponding quiescent values of I_bias in the amplifier stage 10 of FIG. 1.

As discussed above in conjunction with FIGS. 1-2, after the negative transient overshoot caused by the transition of V_transmission from V_high to V_low, I_bias in the amplifier stage 10 settles to a quiescent value that is less than the quiescent value of I_bias when V_transmission=V_high.

Similarly, referring to FIGS. 6-7, after the negative transient overshoot caused by the transition of transmission from V_high to V_low, I_bias in the amplifier stage 50 may settle to a quiescent value that is less than the quiescent value of I_bias when V_transmission=V_high.

Because the sense transistor 56 is matched to the amplifier transistor 12 and, like the amplifier transistor, has its drain D coupled to V_transmission, after the negative overshoot in I_sense caused by the transition of V_transmission from V_high to V_low, I_sense also settles toward a quiescent value that may be less than the quiescent value of I_sense when V_transmission=V_high.

But this reduction in the quiescent value of I_sense reduces the voltage across the resistor 58, and thus reduces the gate voltage of the reference transistor 42 via the feedback resistor 60.

This reduction in the gate voltage of the reference transistor 42 reduces I_ref, and thus increases the voltage at the reference node 32.

The increased voltage at the reference node 32 increases the gate voltages of the sense transistor 56 and the buffer transistor 46, and thus increases I_sense and V_bias.

The increase in V_bias increases I_bias; therefore, I_bias when V_transmission=V_low is closer to its previous quiescent value (when V_transmission=V_high) than it would be if the DC bias circuit 52 lacked the feedback stage 54.

In summary, the feedback stage 54 allows the DC bias circuit 52 to oppose, and thus lessen, the reduction in the quiescent value of I_bias caused by the high-to-low transition of V_transmission as compared to the corresponding reduction in the quiescent value of I_bias of the amplifier stage 10 (FIG. 1). One can select the parameters of the DC bias circuit 52 to provide the desired quiescent value for I_bias when V_transmission=V_low.

According to a similar feedback analysis, the feedback stage 54 allows the DC bias circuit 52 to oppose, and thus lessen, the increase in I_bias caused by the low-to-high transition of V_transmission as compared to the increase in the quiescent value of I_bias of the amplifier stage 10 (FIG. 1). One can select the parameters of the DC bias circuit 52 to provide the desired quiescent value for I_bias when V_transmission=V_high.

Based on the above analyses and depending on the component values, the transistor dimensions, and the operating parameters of the amplifier stage 50, the DC bias circuit 52 may reduce by 66% or more the difference between the quiescent values of I_bias at V_transmission=V_low=1 V and V_transmission=V_high=3.6 V as compared to the difference between the quiescent values of I_bias of the amplifier stage 10 (FIG. 1) for the same values of V_transmission.

Still referring to FIG. 6 and as discussed below, the DC bias circuit 52 may also reduce the difference between the value of I_bias predicted by equations (1) and (2) and the actual value of I_bias

As discussed above in conjunction with FIGS. 1, 2, and 5, for a given value of I_ref, the actual ratio S_actual=I_bias/I_ref may be greater by 80% or more than the predicted ratio S_predicted=I_bias/I_ref for the known amplifier stage 10. This difference between S_predicted and S_actual indicates that for the amplifier stage 10, the actual value of I_bias is significantly greater than the value predicted by equations (1) and (2).

One way to reduce the difference between the actual and predicted values of I_bias is to increase the drain voltage of the reference transistor 42 so that the reference transistor operates in its linear region, not in its triode region.

But when the amplifier stage 50 is used in a low-voltage application such as the transmitter application discussed above, increasing the drain voltage of the reference transistor 42 may not be a viable option.

Another way to reduce the difference between the actual and predicted values of I_bias is to generate a voltage offset between the gate-to-source voltages of the amplifier and reference transistors 12 and 42.

This solution is unavailable in the known bias circuit 14 (FIG. 2) because the gates G and sources S of the amplifier and reference (FIG. 1) transistors 12 and 42 are respectively coupled to the same nodes 32 and ground.

But referring to FIG. 6, the feedback resistor 62 in the DC bias circuit 52 provides a level of decoupling between the gates G of the amplifier and reference transistors 12 and 42, and thus allows the DC bias circuit to generate a voltage offset between the gate-to-source voltages of the amplifier and reference transistors. As discussed below, one can design the DC bias circuit 52 so that this voltage offset brings the actual value of I_bias closer to the value of I_bias predicted by equations (1) and (2).

Specifically, referring to FIG. 6 and Tables V and VII, because the current I₅₈ is greater than I_buffer and because the resistors 44 and 58 have equal values, the voltage across the resistor 44 is less than the voltage across the resistor 58.

This voltage difference generates a positive value for a current I_feedback, which flows from the source S of the sense transistor 56, through the feedback resistors 60 and 62, to the bias node 38.

Therefore, I_feedback causes the gate-to-source voltage of the reference transistor 42 to be greater than V_bias, which is the gate-to-source voltage of the amplifier transistor 12. That is, I_feedback effectively lowers V_bias relative to the gate-to-source voltage of the reference transistor 42.

This lowering of V_bias relative to the gate-to-source voltage of the reference transistor 42 lowers I_bias relative to I_ref, and thus brings I_bias closer to the value predicted by equations (1) and (2).

FIG. 9 is a plot of the actual ratio s_actual=I_bias/I_ref versus I_ref for the above-described embodiment of the amplifier stage 50 of FIG. 6.

As shown in FIG. 9, for 0.1 milliampere (mA)≦I_ref≦4.5 mA, 13≧s_actual≧12.2, which is a maximum increase in s_actual of approximately 34% from the value of s_predicted≈10 given by equation (2). This is compared to a maximum increase of s_actual approximately 80% from s_predicted for the amplifier 10 (FIG. 1) operating under similar conditions as shown in FIG. 5.

Therefore, an embodiment of the DC bias circuit 52 of FIG. 6 brings the actual value of I_bias closer to the value predicted by equations (1) and (2) as evidenced by the approximately 50% reduction in the maximum difference between s_actual and s_predicted as compared to the difference between s_actual and S_predicted for the DC bias circuit 14 of FIG. 1.

If the DC bias circuit 52 of FIG. 6 is to be used solely for the purpose of obtaining an accurate bias current in transistor 12, and no switching of V_transmission is anticipated, then a further embodiment is possible. This is can be done by simplifying the feedback stage 54. Because the feedback stage 54 includes a source follower (i.e., the transistor 56), its output ideally follows its input. In other words, the feedback voltage at the source S of transistor 56 follows closely the voltage at its gate, which is connected to node 32. Therefore an alternate embodiment can be effected by eliminating transistor 56 entirely, connecting node 32 directly to the end of resistor 60 that is at its juncture with resistor 58, and eliminating resistor 58. The only adjustment to resistor 60 might be to increase its value from 100Ω to, e.g., 5.0 KΩ, to recover the nominally high input impedance of the removed source-follower transistor 56. Without resistor 58, the reference node 32 is still higher in voltage than the bias node 38, thus enabling I_feedback to flow and raise gate G of transistor 42 above the V_bias of transistor 12 at bias node 38. The resulting bias difference at the gates G of transistors 12 and 42 may reduce the difference between s_actual and s_predicted as well as the unmodified feedback stage 54 does.

Referring to FIG. 6, alternate embodiments of the amplifier stage 50 and the DC bias circuit 52 are contemplated. For example, one can form the dual of the amplifier 50 by replacing the transistors 12, 42, 46, and 56 with p-channel transistors, and by reversing the polarities of the voltages V_transmission, V_buffer, and V_ref. Furthermore, one can couple the sources S of the transistors 12 and 42 and couple the resistors 44 and 58 to voltages other than ground (0 V). Moreover, the DC bias circuit 52 may include components other than those shown, such as one or more capacitors in parallel with respective ones of the resistors. In addition, the transistors 12, 42, 46, and 56 may be other than GaAs transistors. Furthermore, one can change one or more of the transistor dimensions, component values, and voltage values shown in the Tables IV-VI to obtain different quiescent values for I_ref and I_bias. Moreover, one or more of the transistors 12, 42, 46, and 56 may be a bipolar transistor. In addition, the amplifier stage 50 may be used in systems other than cell phones and wireless modems.

From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated.

Claims

1. A circuit for biasing an amplifier transistor, the circuit comprising: a reference node operable to receive a reference current;a bias node operable to be coupled to the transistor and to carry a bias signal;a buffer stage having a buffer input node coupled to the reference node and having a buffer output node coupled to the bias node;a reference stage coupled to the reference and bias nodes and operable to generate the bias signal from the reference current, the bias signal operable to cause the transistor to conduct a bias current that is proportional to the reference current; anda feedback stage coupled between the reference and bias nodes.

2. The circuit of claim 1, further comprising: a supply node; anda current source coupled between the supply node and the reference node and operable to generate the reference current.

3. The circuit of claim 1, further comprising: a supply node; andan impedance element coupled between the supply node and the reference node and operable to conduct the reference current.

4. The circuit of claim 1, further comprising: a supply node; andwherein the buffer stage comprises a buffer transistor having a first conduction node coupled to the supply node, a second conduction node coupled to the buffer output node, and a control node coupled to the buffer input node.

5. The circuit of claim 1, further comprising: a supply node; andwherein the reference stage comprises, a reference transistor having a first conduction node coupled to the reference node, a second conduction node coupled to the supply node, and a control node coupled to the bias node; andan impedance element serially coupled between the bias node and the supply node.

6. The circuit of claim 1 wherein: the reference stage comprises a control node; andthe feedback stage is coupled to the control node.

7. The circuit of claim 1, further comprising: first and second supply nodes;wherein the reference stage comprises, a reference transistor having a first conduction node coupled to the reference node, a second conduction node coupled to the first supply node, and a control node, anda first impedance element serially coupled between the bias node and the first supply node; andwherein the feedback stage comprises, a feedback transistor having a first conduction node coupled to the second supply node, a second conduction node, and a control node coupled to the reference node,a second impedance element coupled between the second conduction node of the feedback transistor and the first supply node,a third impedance element coupled between the second conduction node of the feedback transistor and the control node of the reference transistor, anda fourth impedance element coupled between the control node of the reference transistor and the bias node.

8. The circuit of claim 1, further comprising: first, second, third, and fourth supply nodes, the amplifier transistor operable to be coupled to the first supply node;a current source coupled between the second supply node and the reference node and operable to generate the reference current;wherein the buffer stage comprises a buffer transistor having a first conduction node coupled to the third supply node, a second conduction node coupled to the buffer output node, and a control node coupled to the buffer input node;wherein the reference stage comprises, a reference transistor having a first conduction node coupled to the reference node, a second conduction node coupled to the fourth supply node, and a control node, anda first impedance element serially coupled between the bias node and the fourth supply node; andwherein the feedback stage comprises, a feedback transistor having a first conduction node coupled to the first supply node, a second conduction node, and a control node coupled to the reference node,a second impedance element coupled between the second conduction node of the feedback transistor and the fourth supply node,a third impedance element coupled between the second conduction node of the feedback transistor and the control node of the reference transistor, anda fourth impedance element coupled between the control node of the reference transistor and the bias node.

9. The circuit of claim 1, further comprising: first, second, third, and fourth supply nodes, the transistor operable to be coupled to the first supply node;a current source coupled between the second supply node and the reference node and operable to generate the reference current;wherein the buffer stage comprises a buffer field-effect transistor having a first conduction node coupled to the third supply node, a second conduction node coupled to the buffer output node, and a control node coupled to the buffer input node;wherein the reference stage comprises, a reference field-effect transistor having a first conduction node coupled to the reference node, a second conduction node coupled to the fourth supply node, and a control node, anda first impedance element serially coupled between the bias node and the fourth supply node; andwherein the feedback stage comprises, a feedback field-effect transistor having a first conduction node coupled to the first supply node, a second conduction node, and a control node coupled to the reference node,a second impedance element coupled between the second conduction node of the feedback transistor and the fourth supply node,a third impedance element coupled between the second conduction node of the feedback transistor and the control node of the reference transistor, anda fourth impedance element coupled between the control node of the reference transistor and the bias node.

10. The circuit of claim 1, further comprising: first, second, third, and fourth supply nodes, the amplifier transistor operable to be coupled to the first supply node;a first resistive element coupled between the second supply node and the reference node and operable to conduct the reference current;wherein the buffer stage comprises a buffer transistor having a first conduction node coupled to the third supply node, a second conduction node coupled to the buffer output node, and a control node coupled to the buffer input node;wherein the reference stage comprises, a reference transistor having a first conduction node coupled to the reference node, a second conduction node coupled to the fourth supply node, and a control node, anda second resistive element serially coupled between the bias node and the fourth supply node; andwherein the feedback stage comprises, a feedback transistor having a first conduction node coupled to the first supply node, a second conduction node, and a control node coupled to the reference node,a third resistive element coupled between the second conduction node of the feedback transistor and the fourth supply node,a fourth resistive element coupled between the second conduction node of the feedback transistor and the control node of the reference transistor, anda fifth resistive element coupled between the control node of the reference transistor and the bias node.

11. The circuit of claim 1 wherein the feedback stage includes a sense element that is operable to: sense a change in the bias current through the amplifier transistor; andcause the reference stage to oppose the change in the bias current by adjusting the bias signal.

12. The circuit of claim 1 wherein the feedback stage is operable to cause the reference stage to oppose a change in the bias signal.

13. A system, comprising: an output node;an amplifier transistor having a first conduction node coupled to the output node, having a control node, and operable to conduct a bias current; anda bias circuit including, a bias node coupled to the control node of the amplifier transistor,a reference node operable to receive a reference current,a buffer stage having a buffer input node coupled to the reference node and having a buffer output node coupled to the bias node,a reference stage coupled to the reference and bias nodes and operable to generate on the bias node and from the reference current a bias signal operable to cause the bias current to be proportional to the reference current, anda feedback stage coupled between the reference and bias nodes.

14. The system of claim 13 wherein the amplifier transistor comprises a GaAs field-effect transistor.

15. The system of claim 13, further comprising: first, second, third, and fourth supply nodes;a first impedance element coupled between the first supply node and the output node;wherein the amplifier transistor has a second conduction node coupled to the second supply node;wherein the buffer stage is coupled to the third supply node;wherein the reference stage is coupled to the fourth supply node; andwherein the feedback stage is coupled to the first supply node.

16. The system of claim 13, further comprising a load coupled to the output node.

17. The system of claim 13, further comprising: a signal generator operable to provide a signal on the control node of the drive transistor; andan antenna coupled to the output node and operable to transmit the signal.

18. A method, comprising: establishing a bias current through an amplifier transistor;sensing a change in the bias current; andadjusting the bias current in a direction opposite to the change.

19. The method of claim 18, further comprising: wherein establishing the bias current comprises, generating a first bias voltage between a control node and a first conduction node of the amplifier transistor that is coupled to a supply voltage, andgenerating a second bias voltage between the control node and a second conduction node of the amplifier transistor;establishing a sense current through a sense transistor by, generating approximately the first bias voltage between a control node and a first conduction node of a sense transistor that is coupled to the supply voltage, andgenerating approximately the second bias voltage between the control node and a second conduction node of the sense transistor;wherein sensing change in the bias current comprises sensing in the sense current a change that is proportional to the change in the bias current; andwherein adjusting the bias current comprises adjusting a voltage on the control node of the drive transistor in response to the sensed change in the sense current.

20. A method, comprising: generating a reference signal that is proportional to a reference current flowing through a reference transistor;generating a bias signal from the reference signal, the bias signal being unequal to the reference signal; andestablishing a bias current through an amplifier transistor by coupling the bias signal to the amplifier transistor.