Negative gain transductance amplifier circuit

Abstract

The invention proposes a negative gain transconductance amplifier circuit (1) for capacitive load that includes: an RC serial circuit connected between an input terminal (E) of the amplifier circuit and a intermediate terminal (A); an amplification level connected between the intermediate terminal and an output terminal (S) designed to be connected to a capacitive load, and which includes: a first negative gain transconductance amplifier (2) connected via open loop between the intermediate terminal and the output terminal; a second negative gain transconductance amplifier (3) with characteristics that are notably identical to those of the first amplifier, connected via closed loop; its input and output are connected to the intermediate terminal via a resistance (R1).

Description

This invention relates to linear amplifiers, in particular negative gain transconductance amplifier circuits.

The document Patent Abstracts of Japan, Vol 014, # 537 (E-1006), dated Nov. 27, 1990 describes an amplifier circuit that includes a first closed loop inverter with a first resistance, connected to a second open loop inverter using a second resistance. The values of the first and second resistances are the same.

This amplifier circuit is not optimised and has limited passband.

FIG. 1 illustrates a known example of an integrated linear amplifier 1 connected to a capacitive load C_L. Amplifier circuit 1 comprises a CMOS inverter 2 described notably in U.S. Pat. No. 4,178,605. Inverter 2 includes a PMos transistor 21 and an NMos 22 transistor. The source of transistor 21 is connected to a VDD power supply voltage that is sufficiently high for the operating regime of transistors 21 and 22. The source of transistor 22 is grounded. The grids of transistors 21 and 22 are connected to node A of inverter 2. The drains of transistors 21 and 22 are connected to the output S of amplifier circuit 1 and has a capacitive load C_L. In a manner that is known per se, amplifier circuit 1 has a resistance R1 connected between A and S, an input resistance Rin and a capacitance C1 serially connected between input E and node A.

When transistors 21 and 22 have an identical channel length (L_P and L_N), the auto-polarisation of inverter 2 results in a static polarisation voltage at node A of approximately VDD/2 if the following condition is verified:W_P/W_N=μ_N/μ_P

Where W_P and W_N are the respective channel widths of transistors 21 and 22, and μ_N and μ_P are their respective mobilities.

At the polarisation point, V_A=V_S=V_DD/2, the transistors 21 and 22 are operating in the saturation regime. The amplitude of the voltage transfer A_V of the amplifier circuit 1 in its passband is therefore approximately as follows:A_V=V_s/V_g˜−g_m*(R₀*R₁)/(R₀+R₁)

where g_m is the sum of the respective transconductances of transistors 21 and 22;

R₀ is the output resistance of the inverter.

R₀ is defined by R₀=1/(g_DSN+g_DSP). g_DSP and g_DSN are the respective output conductances of transistors 21 and 22.

For a high value of A_V, the input impedance of the amplifier circuit is defined by:Z_E˜R_in−R₁/A_V

For a high value of AV, the output impedance of the amplifier circuit is defined by:Z_S˜R₁*R₀/(R₁+R₀)

For a high value of A_V, the low cut-off frequency Fcb and the high cut-off frequency Fch are defined, respectively, by the following formulas:Fcb˜−A_V/(2π*R₁*C₁)Fch˜1/(2π*(R₁*R₀)/(R₁+R₀)*C_L)

In practice, there is a need for a voltage transfer of the increased amplifier circuit, for increased input and output impedances, and for a reduced cut-off frequency (for an equivalent product of the gain by the passband.)

The invention relates to solving one or several of these disadvantages. The invention thus relates to a negative gain transconductance amplifier circuit for capacitive load that includes:

an RC serial circuit connected between an input terminal of the amplifier circuit and an intermediate terminal;

an amplification stage connected between the intermediate terminal and an output terminal intended to be connected to a capacitive load, which includes:

a first negative gain transconductance amplifier connected via open loop between the intermediate terminal and the output terminal;

a second negative gain transconductance amplifier with characteristics that are essentially identical to those of the first amplifier, connected via closed loop; its input and output are connected to the intermediate terminal via a resistance.

According to one variation, the second amplifier is configured to impose a static polarisation voltage from the first amplifier on the intermediate terminal that is equal to half the power supply voltage of the first amplifier.

According to another variant, the amplifier circuit includes a capacitive load connected to the output terminal.

According to yet another variant, the first and second negative gain transconductance amplifiers include an NMos transistor and a PMos transistor where their grids are connected at the amplifier input and their drains are connected to the amplifier output; the PMos transistor source is connected to a power supply voltage and the NMos transistor source is connected to a ground voltage.

We can therefore expect that the NMos transistors of the first and second amplifiers have a channel length that is essentially identical, a channel width that is essentially identical, and a mobility that is essentially identical and that the PMos transistors of the first and second amplifiers have a channel length that is essentially identical, an identical channel width, and an identical mobility.

According to one variant, the PMos transistors and the NMos transistors are matched.

According to yet another variant, the resistance between the intermediate terminal and the input of the second amplifier includes a transistor that is kept on by a static polarisation voltage.

The invention also relates to an amplifier circuit that includes:

the amplifier circuit in which the first and second negative gain transconductance amplifiers include an NMos transistor and a PMos transistor;

a circuit that applies a test signal on the input terminal of the amplifier circuit;

a counter that increments when a blocking signal is absent;

a current source that applies a current to the PMos transistor source from the first amplifier; the current applied increases as the counter increases;

a comparator that has a first input connected to the output terminal of the amplifier circuit and a second input that receives a reference level; the comparator applies a blocking signal to the counter when the first input level is greater than the second input level.

The invention also relates to a radiofrequency reception circuit that has a homodyne reception chain equipped with a mixer, and an amplifier circuit such that the input terminal is connected to the output of the mixer.

Other features and advantages will become evident from the description that follows, provided by way of indication and in no way restrictive, with reference to the attached figures, in which:

FIG. 1 illustrates an example of a negative gain transconductance amplifier circuit according to the prior art;

FIG. 2 illustrates an example of a negative gain transconductance amplifier circuit according to the invention;

FIG. 3 illustrates a first variant of the amplifier circuit of FIG. 2;

FIG. 4 illustrates a second variant of the amplifier circuit of FIG. 2;

FIG. 5 compares the loadless voltage transfer amplitudes between the amplifier circuits of FIGS. 1 and 3;

FIG. 6 compares the voltage transfer amplitudes under capacitive load of the amplifier circuits of FIGS. 1 and 3;

FIG. 7 compares the loadless transient responses of the amplifier circuits of FIGS. 1 and 3 in their passband;

FIG. 8 illustrates an example of a circuit incorporating the amplifier circuits according to the invention;

FIG. 9 illustrates a transmission/reception chain incorporating the circuit of FIG. 8.

The invention proposes a negative gain transconductance amplifier circuit for capacitive load that presents a modified amplification stage. A first negative gain transconductance amplifier is intended to amplify the input signal and is connected in open circuit. A second amplifier has characteristics that are essentially identical to those of the first and is connected in closed circuit and its input and output are connected to the input of the first amplifier via a resistance.

The absolute value of the amplification gain is thus increased, the size of the capacitive load can be increased for a given gain, the low cut-off frequency of the amplifier is lowered as well as the distortion level and the gain passband product is maintained. The circuit according to the invention thus combines the advantages of an open-loop amplifier and the advantages of a closed-loop amplifier.

FIG. 2 represents an example of an amplifier circuit according to the invention. A serial RC circuit is connected between an input terminal E and an intermediate terminal A. The capacitance C₁ allows filtering the continuous component of the signal applied to terminal E. The amplification stage, connected between the intermediate terminal A and an output terminal S includes a first negative gain transconductance amplifier 2 connected via open-loop between terminals A and S. The amplification stage also includes a second negative gain transconductance amplifier 3 connected via closed loop; its input and output are connected to terminal A via the resistance R1. Amplifier 3 has characteristics that are essentially identical to those of amplifier 2. The amplifiers therefore have the same common mode. A capacitive load C_L is connected between the output terminal S and a ground.

Amplifier 3 is configured to impose a static polarisation voltage of amplifier 2 equal to half the power supply voltage of amplifier 2. Around this polarisation point, amplifier 2 behaves as a voltage controlled current source.

By essentially identical characteristics of amplifiers 2 and 3, is understood identical technology, essentially identical components, having the same structure and the same voltage levels. The illustrated amplifiers 2 and 3 only differ in the connections of their inputs and outputs.

The circuit in FIG. 3 uses amplifiers 2 and 3, each of which includes a CMOS inverter. As in the example in FIG. 1, each CMOS inverter includes an NMos transistor and a PMos transistor where their grids are connected to the amplifier input, and their drains are connected to the amplifier output; the PMos transistor source is connected to a V_DD power supply voltage and the NMos transistor source is connected to ground.

The NMos transistors of amplifiers 2 and 3 have an identical channel length, an identical channel width, and identical mobility within the manufacturing deviations. Similarly, the PMos transistors of amplifiers 2 and 3 have an identical channel length, an identical channel width, and identical mobility within the manufacturing deviations. Preferably, the NMos transistors of amplifiers 2 and 3 are matched in order to improve the adaptation of their common modes and thus increase the amplification gain absolute value Similarly, the PMos transistors of these amplifiers are matched.

The amplifier circuit therefore presents the following characteristics for |A_V|>>1:A_V˜−g_m*R₀

With respect to the solution in FIG. 1, the gain is therefore theoretically increased by the order of (R₀+R₁)/R₁.

The input and output impedances in the passband are defined by the following formulas:Z_E=R₁+(R_M//C_G)Z_X=R₀

Where R_M is defined by 1/(g_mN+g_mP), g_mN and g_mP are the respective output transconductances of the NMos and PMos transistors in saturation regime, C_G is the input parasite capacitance of amplifiers 2 and 3, (R_M//C_G) is the impedance equivalent to placing R_M and C_G in parallel.

The low and high cut-off frequencies (for R₀>>R_M) are therefore defined by:Fcb˜1/(2π*R₁*C₁)Fch˜1/(2π*R₀*C_L)

The low cut-off frequency is therefore approximately reduced by a factor that is essentially equal to the A_V voltage transfer amplitude.

Preferably, resistance R1 is comprised of a transistor 4 connected between terminal A and the amplifier 3 input. The transistor is kept on in linear regime by a static polarisation voltage. Transistor 4 is, for example, an NMos or PMos transistor. In the example in FIG. 4, the transistor 4 grid is supplied by a Vbias voltage; the NMos source is connected to terminal A and its drain is connected to the amplifier 3 input and output.

Performance comparisons between the amplifiers in FIGS. 1 and 3 have been simulated and are illustrated in FIGS. 5 to 7. The parameters used for the simulation are the following:R1=30Ω, Rin=200Ω, and C1=20 pF;W_N/L_N=3, μ_N=0.35; W_P/L_P=9, μ_P=0.35;

		FIG. 1
	Proposed	Topology
Parameters	Topology	Ref [1]	Unit
VDD power supply voltage	2.5	V
VSS = gnd power supply voltage	0	V
Polarisation voltage at	1.285	V
output
NMOS transconductance (gmN)	814.4μ	Ω−1
NMOS conductance (gDSN)	25μ	Ω−1
PMOS transconductance (gmP)	950μ	Ω−1
PMOS conductance (gDSP)	30μ	Ω−1
Input impedance in band	30 k	1.7 k	Ω
(module)
Output resistance	19.48 k	10.4 k	Ω
Total harmonic distortion	5.5	3.94	%
rate (1 VCC at loadless output)
Total consumption	1927	964	μW
CL˜16 fF loadless simulation
VS/VE voltage gain in band	−31.5	−16.7	V/V
Product (passband * gain)	17.2	16	GHz
(−3 dB)
Low cut-off frequency (−3 dB)	0.251	4.62	MHz
High cut-off frequency (−3 dB)	549	965	MHz
Simulation for CL = 0.5 pF
VS/VE voltage gain in band	−31	−16.6	V/V
Passband * gain product)	504	495	MHz
(−3 dB)

FIG. 5 compares the loadless voltage transfer amplitudes. Curves 5 and 6 correspond, respectively, to the amplifiers in FIGS. 1 and 3. It can be observed that the low cut-off frequency of the amplifier according to the invention is notably lower than that of the amplifier according to the prior art. The maximum voltage transfer amplitude of the amplifier according to the invention is much greater than that of the amplifier of the prior art. Furthermore, the product of the gain by the passband is essentially maintained.

FIG. 6 compares the voltage transfer amplitudes with a capacitive load. Curves 7 and 8 correspond, respectively, to the amplifiers in FIGS. 1 and 3. Again, it can be observed that the amplifier low cut-off frequency according to the invention is markedly lower than that of the amplifier according to the prior art. The maximum voltage transfer amplitude of the amplifier according to the invention is again much higher than that of the amplifier according to the prior art. The product of the gain by the passband is also essentially maintained.

FIG. 7 compares the loadless transient responses of the amplifiers in FIGS. 1 and 3. It can be observed that the amplifier circuit according to the invention does not create any significant distortions of the amplified signal with respect to the amplifier circuit of the state of the art.

Although we have mainly described an example of a circuit equipped with CMOS type amplifiers, the invention is also applicable to other types of negative gain transconductance amplifiers.

Because of its amplification and passband filtering properties, an amplifier circuit like the one described above is advantageously incorporated into an amplifier circuit that calibrates its passband. FIG. 8 illustrates an amplifier circuit 16 with a calibrated passband incorporating amplifier circuits 1 according to the invention.

Circuit 16 includes a counter 12 that controls the current sources 13. Each time the counter 16 increments, the current set point applied to the current sources 13 increases. Each current source 13 is connected to a respective amplifier circuit 1. The amplifier circuit 1 outputs are connected to an input of the comparator 14. The other input of the comparator 14 receives a passband set point provided by the reference circuit 15.

By using an amplifier circuit 1 according to FIG. 3, the current of a source 13 is injected onto the PMos transistor sources of amplifiers 2 and 3. The passband of amplifier circuit 1 is therefore essentially proportional to the current level applied by the current source.

The calibration phase is performed as follows: the counter is initially set at a low level. The current applied to each source 13 on its amplifier circuit 1 is therefore low. The passband of each amplifier circuit 1 is thus initially reduced. A test signal is applied to the input terminal of each amplifier circuit 1. The signal level on the output terminal of each amplifier circuit 1 is representative of its passband. As long as the signal level on the output terminal is less than the passband set point provided by circuit 15, the output signal of the comparator controls the increase of counter 12. Counter 12 increases and the source 13 current thus increases, until the signal level over the output terminal of an amplifier circuit 1 exceeds the passband set point. The output signal of the comparator thus controls the blocking of counter 12. The passband of the amplifier circuits 1 is thus calibrated.

The test signal applied on the input terminal of the amplifier circuit is, for example, a square wave clock signal. A sinusoidal test signal may also be applied where its frequency is the desired high cut-off frequency. Calibration may be performed at regular intervals in order to compensate for the drift of the amplifier circuit 1 components.

For an amplifier circuit 1 that has CMOS transistor amplifiers, the consumption and size of the transistors can be reduced by calibration: the amplifier only consumes the current that corresponds to the necessary calibrated passband.

Amplifier circuits 1 are particularly adapted for each reception chain since they have a lowered cut-off frequency; their passband can therefore include signals coming from mixers.

Amplifier circuits 16 are therefore advantageously incorporated into the homodyne reception chain. FIG. 9 illustrates a radiofrequency communication circuit 17 with a homodyne reception chain 18 equipped with mixers 19 and 20. The output of each mixer is connected to the input terminal of amplifier circuit 1 of an amplifier circuit 16.

The calibration of an amplifier circuit passband can be performed at regular intervals, for example, every second.

Claims

1. Negative gain transconductance amplifier circuit (1) for capacitive load that includes: an RC serial circuit connected between an input terminal (E) of the amplifier circuit and an intermediate terminal (A); an amplification stage connected between the intermediate terminal and an output terminal (S) intended to be connected to a capacitive load, characterized in that this amplification stage includes: a first negative gain transconductance amplifier (2) connected via open loop between the intermediate terminal and the output terminal; a second negative gain transconductance amplifier (3) with characteristics that are essentially identical to those of the first amplifier, connected via closed loop; its input and output are connected to the intermediate terminal via a resistance (R1, 4).

2. Amplifier circuit (1) according to claim 1, characterized in that the second amplifier (3) is configured to impose a static polarization voltage from the first amplifier on the intermediate terminal (A) that is equal to half the power supply voltage of the first amplifier (2).

3. Amplifier circuit according to claim 1, characterized in that it includes a capacitive load (CL) connected to the output terminal (S).

4. Amplifier circuit according to claim 1, characterized in that the first and second negative gain transconductance amplifiers (2, 3) include an NMos transistor and a PMos transistor where its grids are connected to the amplifier input and where its drains are connected to the amplifier output; the PMos transistor source is connected to a power supply voltage and the NMos transistor source is connected to a ground voltage.

5. Amplifier circuit according to claim 4, characterized in that: the NMos transistors of the first and second amplifiers have a channel length that is essentially identical, a channel width that is essentially identical, and mobility that is essentially identical; the PMos transistors of the first and second amplifiers have a channel length that is essentially identical, an identical channel width, and an identical mobility.

6. Amplifier circuit according to claim 4, characterized in that the PMos transistors and the NMos transistors are matched.

7. Amplifier circuit according to claim 1, characterized in that the resistance between the intermediate terminal and the input of the second amplifier includes a transistor (4) that is kept on by a static polarization voltage.

8. Amplifier circuit (16), characterized in that it includes: an amplifier circuit according to claim 4 or any of its dependent claims; a circuit that applies a test signal on the input terminal (E) of the amplifier circuit; a counter (12) that increments when a block signal is absent; a current source (13) that applies a current to the PMos transistor source of the first amplifier (2); the applied current crosses as the counter increases; a comparator (14) that has a first input connected to the output terminal of the amplifier circuit and a second input that receives a reference level; the comparator applies a blocking signal to the counter (12) when the level of the first input is greater than the level of the second input.

9. Radiofrequency reception circuit (17) that has a homodyne reception chain (18) equipped with a mixer (19, 20), characterized in that it also includes an amplifier circuit according to claim 8 where its input terminal is connected to the output of the mixer.