Single-Stage High-Gain Amplifier

The present invention relates to electronic signal amplifiers and, in particular, to a single-stage solid-state amplifier that provides high gain and has low noise characteristics.

Each amplification stage of a multistage amplifier introduces noise and phase shift into the signal it amplifies. It is, therefore, desirable to use an amplifier having few amplification stages with high gain and low noise. Currently available single-stage amplifiers implemented with short-channel field effect transistors (FETs) provide relatively low amounts of gain or exhibit poor noise performance.

FIG. 1 shows a prior art low-gain amplifier circuit 10 having a single amplification stage comprised of FETs Q.sub.1 and Q.sub.2 connected in cascode arrangement. An input signal, V.sub.i, which is developed from the current delivered by an input current source, I.sub.i, is applied to the gate terminal of Q.sub.1. A FET transistor Q.sub.3 functions as a current source for biasing diodes D.sub.3 and D.sub.4 that ensure a sufficient drain-to-source voltage across Q.sub.1.

A FET Q.sub.4 and diodes D.sub.5, D.sub.6, D.sub.7, D.sub.8, and D.sub.9 form a source follower and DC voltage level shifter. A FET Q.sub.5 functions as a current source for Q.sub.4, and diodes D.sub.1 and D.sub.2 drop a DC voltage, ensuring sufficient drain-to-source voltage for Q.sub.5. A feedback resistor, R.sub.f, connected between the drain terminal of Q.sub.5 and the gate terminal of Q.sub.1 may be used to form a transimpedance amplifier whose bandwidth is dependent on the voltage gain of the amplifier. The bandwidth of such a transimpedance amplifier is approximately equal to A.sub.v /(2.pi.R.sub.f C), where A.sub.v is the voltage gain of the amplifier and C is the total input capacitance. Each of the transistors Q.sub.1, Q.sub.2, and Q.sub.5 is a depletion-mode gallium arsenide (GaAs) metal semiconductor field effect transistor (MESFET).

A resistor R.sub.L, which is represented in FIG. 1 as a variable resistor, is connected between V.sub.dd and the drain terminal of Q.sub.2 to provide a passive load resistance for amplifier 10. A resistive load provides amplifier 10 with lower noise characteristics than those achievable with an active load consisting of a transistor connected as a current source. An active transistor load would typically have channel noise comparable to that of Q.sub.1 and, therefore, degrade the noise performance by approximately 3 dB. On the other hand, a resistive load has thermal noise that is typically three to ten times smaller than the channel noise of a transistor load. The voltage gain, A.sub.v, measured from the gate terminal of Q.sub.1 to the drain terminal of Q.sub.2, is approximately equal to the transconductance g.sub.m of Q.sub.1 multiplied by R.sub.L, as expressed in equation (1) below:

It is known that the transconductance, g.sub.m, is proportional to the square root of the current I.sub.d that enters the drain of Q.sub.1, which is expressed in equation (2), below:

Equation (1) indicates that A.sub.v increases as the value of R.sub.L increases. An increase in the value of R.sub.L causes a corresponding decrease in the value of I.sub.d, which flows through R.sub.L. Equation (2) indicates, however, that a decrease in the value of I.sub.d decreases the value of g.sub.m, thereby counteracting the increase in A.sub.v caused by an increase in the value of R.sub.L. Therefore, according to equations (1) and (2), A.sub.v does not increase linearly with an increase in R.sub.L. As a consequence, the maximum value of A.sub.V is lower than desired. Moreover, as R.sub.L increases in value, the output resistance present at the drain of Q.sub.2 lowers the effective load resistance present at the drain terminal of Q.sub.2, thereby further limiting the overall gain, A.sub.v.

A practical implementation of circuit 10 that provides A.sub.v =-13 would include values of R.sub.L =500 ohms, V.sub.ss =-5 volts, V.sub.dd =+5 volts, and I.sub.d =10 milliamperes. The output impedance, R.sub.O, of common gate stage Q.sub.2 would be approximately 3.25 kilohms. Transistor Q.sub.1 would have g.sub.m =30 millimhos, gate width=500 microns, f.sub.T =15 GHz, and pinchoff voltage V.sub.p =-0.7 volts.

An object of the invention is, therefore, to provide a single-stage amplifier with high gain and low noise characteristics.

A further object of the invention is to provide a single-stage, high-gain amplifier in a short-channel, high-speed technology such as GaAs that can be suitably employed as a transimpedance amplifier.

A high-gain solid-state amplifier includes an amplification stage with a transistor having a current dependent transconductance value. The transistor is operatively connected to a load resistor through which a load current flows. The value of the load resistor together with the transconductance value of the transistor affects the voltage gain of the amplifier. A bias resistor provides a supplemental bias current to a current summing node. The current summing node sums the load current and the supplemental bias current and provides the transistor with a total current that determines the transconductance value. The value of the supplemental bias current is chosen to supplement the load current to provide a predetermined total current so that the voltage gain may be adjusted by adjusting the load resistor without changing the transconductance value of the transistor.

Alternatively, the amplifier voltage gain may be enhanced by increasing the supplemental bias current and transistor size in a known manner while maintaining a fixed value of the load resistor.

FIG. 1 shows a prior art single-stage amplifier circuit.

FIG. 2 shows a high-gain, low-noise single-stage amplifier circuit configured in accordance with the present invention.

FIG. 2 shows a high-gain, low-noise amplifier circuit 20 configured in accordance with the present invention. Amplifier 20 may be used, for example, as the input amplification stage of a very high-gain multi-stage amplifier or as a single-stage transimpedance amplifier. Transistors Q.sub.1, Q.sub.2, Q.sub.4, and Q.sub.5 are of the same type shown in FIG. 1. The implementation of the invention is not, however, limited to GaAs devices or any other particular type of transistor. A resistor R.sub.3, which is connected between V.sub.dd and the gate terminal of Q.sub.2, replaces the transistor Q.sub.3 configured as an active load in FIG. 1.

A bias resistor R.sub.2 and a transistor Q.sub.12 , which is preferably of the enhancement mode MESFET type, are added to amplifier circuit 10 in FIG. 1. Diodes D.sub.3 and D.sub.4, which are reconfigured to accommodate the addition of Q.sub.12 , shift the DC voltage levels at the source terminals of Q.sub.1 and Q.sub.2 to provide the proper operating drain-to-source voltages for Q.sub.1 and Q.sub.12. Amplifier circuit 20 operates in a manner that substantially eliminates the dependence of I.sub.d on the value of R.sub.L1 and increases the impedance looking into the drain of Q.sub.2, thereby permitting the selection of R.sub.L1 without affecting the value of g.sub.m for Q.sub.1.

As shown in FIG. 2, a current I.sub.d flows into the drain of Q.sub.1 ; a current I.sub.1 flows through resistor R.sub.L1, which is connected between V.sub.dd and the drain of Q.sub.2 ; and a current I.sub.2 flows through resistor R.sub.2, which is connected between V.sub.dd and the drain of Q.sub.1. Because of the relatively high input impedance of Q.sub.12, current I.sub.d is essentially equal to I.sub.1 +I.sub.2. Thus, I.sub.d flows out of node "A" and I.sub.1 +I.sub.2 flows into node "A."

A negative feedback loop is created by the connection of the gate and drain terminals of Q.sub.12 to the respective source and gate terminals of Q.sub.2 as shown. This feedback loop provides a low input impedance at node "A" and a high output impedance, R.sub.0, into the drain of Q.sub.2. The presence of Q.sub.12 causes the output impedance R to be comparable to or greater than R.sub.L1, notwithstanding the presence of R.sub.2. The absence of Q.sub.12 would result in R.sub.2 lowering the value of the impedance presented at the drain of Q.sub.2 and, therefore, the voltage gain A.sub.v, of the amplifier. If R.sub.2 were replaced by a higher impedance current source transistor, the noise performance of the circuit would be degraded. Resistor R.sub.2 provides a current I.sub.2 that affects the value of I.sub.d and the value of g.sub.m of Q.sub.1. Therefore, selecting a value of R.sub.L1, and thereby changing the value of I.sub.1, need not appreciably change the value of I.sub.d. This is so because the value of R.sub.2 can be changed to provide a corresponding change in I.sub.2 that offsets a change in I.sub.1 and thereby maintains I.sub.d and g.sub.m at constant values. The result is that the gain, A.sub.v, from the gate of Q.sub.1 to the drain of Q.sub.2 can be made much larger than that of amplifier circuit 10 of FIG. 1.

The gate width of transistor Q.sub.2 is typically made much smaller than that of Q.sub.1, such that the output impedance of Q.sub.2 is correspondingly higher. The impedance R.sub.0 is approximately inversely proportional to the width of Q.sub.2 and is proportional to the voltage gain provided by Q.sub.12.

The voltage gain, A.sub.v, of amplifier 20 is approximately equal to g.sub.m of Q.sub.1 multiplied by R.sub.L1 in parallel with R.sub.0, as expressed in equation (3), below:

In equation (3), R.sub.0 is made comparable to or larger than R.sub.L1, which is made large to increase the voltage gain, A.sub.v. The negative feedback provided by Q.sub.12 keeps R.sub.0, and thus A.sub.v, high. The value of R.sub.0 in the present invention is typically five to ten times that achievable by the prior art circuit 10. The gain, A.sub.v, from the gate of Q.sub.1 to the drain of Q.sub.2 can be greater than 100 for the component values given with reference to amplifier circuit 10, and for R.sub.2 =850 ohms, R.sub.3 =2.5 kilohms, I.sub.d =10 milliamperes, I.sub.1 =1 milliamperes, and R.sub.L1 =5 kilohms. Without Q.sub.12, R.sub.2 would lower R.sub.0 and thereby counteract the increase in R.sub.L1 and therefore A.sub.v.

Although the preferred operation entails the use of positive and negative power supplies for V.sub.dd and V.sub.ss, respectively, the performance of amplifier circuit 20 is also superior to that of amplifier circuit 10 when biased by a single power supply.

Amplifier 20 may be also used with a feedback resistor, R.sub.f, as indicated previously with reference to amplifier circuit 10. By using resistors R.sub.L1, R.sub.2, and R.sub.3 rather than active load/biasing transistor current sources, the noise performance of the amplifier 20 is enhanced. This increases the sensitivity for transimpedance amplifier applications. For transimpedance amplifiers with wide bandwidth and high sensitivity, it is desirable to achieve a high voltage gain in a single stage. This minimizes phase shift and thereby enhances stability and allows the use of a large feedback resistor, R.sub.f.

Transistor Q.sub.12 could also be a depletion device, which may require that extra diodes be connected between the source of Q.sub.12 and the anode of D.sub.3, and between the source of Q.sub.2 and the anode of D.sub.4. These diodes would ensure sufficient drain-to-source voltage across Q.sub.1 and Q.sub.12.

While the present invention has been described with respect to a preferred embodiment, it will be understood that various alterations may be made thereto without departing from the scope of the invention. For example, the gain, A.sub.v, can be increased by keeping R.sub.L1 and I.sub.1 at constant values and increasing the value of current I.sub.2 and the ratio of the width to the length of Q.sub.1. Ordinarily, the gate length is chosen to be as small as possible and the gate width is selected as needed. This approach would also provide an independent adjustment of the load applied to the drain terminal of Q.sub.2 and the value of g.sub.m for Q.sub.1 without substantially raising the output impedance of Q.sub.2. The scope of the invention should, therefore, be determined only by the following claims.