Feedback Amplifiers part3

Reduction in Nonlinear Distortion

Curve (a) in Fig. 5.3 shows the transfer characteristic of an amplifier. As indicated, the characteristic is piecewise linear, with the voltage gain changing from 1000 to 100 and then to 0.This nonlinear transfer characteristic will result in this amplifier generating a large amount of nonlinear distortion.

The amplifier transfer characteristic can be considerably linearized (i.e., made less nonlinear) through the application of negative feedback. Thus large changes in open-loop gain (1000 to 100 in this case) give rise to much smaller corresponding changes in the closed-loop gain.

It should be noted that negative feedback can do nothing at all about amplifier saturation, since in saturation the gain is very small (almost zero) and hence the amount of feedback is also very small (almost zero).

Figure 5.3 Illustrating the application of negative feedback to reduce the nonlinear distortion in amplifiers. Curve (a) shows the amplifier transfer characteristic without feedback. Curve (b) shows the characteristic with negative feedback (b = 0.01) applied.

THE FOUR BASIC FEEDBACK TOPOLOGIES

Based on the quantity to be amplified (voltage or current) and on the desired form of output (voltage or current), amplifiers can be classified into four categories. In the following, we shall review this amplifier classification and point out the feedback topology appropriate in each case.

Voltage Amplifiers

Voltage amplifiers are intended to amplify an input voltage signal and provide an output voltage signal. The voltage amplifier is essentially a voltage-controlled voltage source. The input impedance is required to be high, and the output impedance is required to be low. Since the signal source is essentially a voltage source, it is convenient to represent it in terms of a Thevenin equivalent circuit. In a voltage amplifier the output quantity of interest is the output voltage. It follows that the feedback network should sample the output voltage. Also, because of the Thevenin representation of the source, the feedback signal should be a voltage that can be mixed with the source voltage in series.

A suitable feedback topology for the voltage amplilier is the voltage-mixing voltage-sampling one shown in Fig. 5.4(a). Because of the series connection at the

input and the parallel or shunt connection at the output, this feedback topology is also known as series-shunt feedback. As will be shown, this topology not only stabilizes the voltage gain but also results in a higher input resistance (intuitively, a result of the series connection at the input) and a lower output resistance (intuitively, a result of the parallel connection at the output), which are desirable properties for a voltage amplifier. The noninverting op-amp configuration of Fig. E5.1 is an example of series-shunt feedback.

Current Amplifiers

The input signal in a current amplifier is essentially a current, and thus the signal source is most conveniently represented by its Norton equivalent. The output quantity of interest is current; hence the feedback network should sample the output current. The feedback signal should be in current form so that it may be mixed in shunt with the source current. Thus the feedback topology suitable for a current amplifier is the current-mixing current-sampling topology, illustrated in Fig. 5.4(b). Because of the parallel (or shunt) connection at the input, and the series connection at the output, this feedback topology is also known as shunt-series feedback. As will be shown, this topology not only stabilizes the current gain but also results in a lower input resistance, and a higher output resistance, both desirable properties for a current amplifier.

An example of the shunt-series feedback topolog iven in Fig. 5.5. at the bias details are not s Also note that the current g sampled is not put current, but the eq wing from the source of he referen indicated in Fig. 5.5 for eedback current ch that it subtracts fr all circuits, therefore, for the feedback to gative, the loop gain positive.

Following the signal around the loop, for instance, let the current in Fig. 5.5 increase. We see that the gate voltage of will increase, and thus its drain current will also increase. This will cause the drain voltage of , (and the gate voltage of to decrease, and thus the drain current of , will decrease. From the feedback network we see that if decreases, then (in the direction shown) will increase. The increase in will subtract from , causing a smaller increment to be seen by the amplifier. Hence the feedback is negative.

Transconductance Amplifiers

In transconductance amplifiers the input signal is a voltage and the output signal is a current. It follows that the appropriate feedback topology is the voltage-mixing currentsampling topology, illustrated in Fig. 5.4(c). The presence of the series connection at both the input and the output gives this feedback topology the alternative name seriesseries feedback. An example of this feedback topology is given in Fig. 5.6. Here, note that as in the circuit of Fig. 5.5 the current sampled is not the output current but the almost-equal emitter current of . In addition, the mixing loop is not a conventional one; it is not a simple series connection, since the feedback signal developed across is in the emitter circuit of , while the source is in the base circuit of . These two approximations are done for convenience of circuit design.

Transresistance Amplifiers

In transconductance amplifiers the input signal is a voltage and the output signal is a current. It follows that the appropriate feedback topology is the voltage-mixing current- sampling topology, illustrated in Fig. 5.4(c). The presence of the series connection at both the input and the output gives this feedback topology the alternative name series- series feedback.

An example of this feedback topology is found in the inverting op-amp configuration of Fig,5.7(a). The circuit is redrawn in Fig. 5.7(b) with the source converted to Norton's form.