Voltage and current feedback amplifiers are almost identical

The construction of voltage and current feedback amplifier application circuits is generally the same, except for a few key points.

Current feedback amplifiers have higher slew rates than voltage feedback amplifiers. As such, current feedback amplifiers are better able to solve high-speed problems than voltage feedback amplifiers. The name "current feedback amplifier" carries some mystery, but the general structure of current feedback and voltage feedback amplifier application circuits is the same, except for a few key points.

First, the feedback resistor value of a current feedback amplifier circuit must be kept within a small range. The lower the resistor value, the worse the stability of the current feedback amplifier. The specified resistor values ​​can be found in the current feedback amplifier product manual. The feedback resistor values ​​of voltage feedback amplifiers are wider. The amplifier's drive capability limits the minimum value of the resistor, and the overall circuit noise limits the maximum value of the resistor.

Figure 1 shows a circuit suitable for either current or voltage feedback amplifiers. If the feedback resistor, RF, equals 2RIN, where RIN is the input resistance, the closed-loop gain of each channel is –2V/V. At first glance, it is easy to assume that the closed-loop bandwidth is equal to the gain-bandwidth product divided by the gain of each channel, or |–2V/V|. But don’t make that assumption!

If a voltage or current feedback amplifier is used as shown in the circuit of Figure 1, the noise gain is:

Where N is the number of input channels. The bandwidth of a circuit with a voltage feedback amplifier is equal to the gain-bandwidth product divided by the noise gain. For example, if a voltage feedback amplifier with a 180MHz gain-bandwidth product is used, three input channels (N=3), and a gain of –2V/V, the circuit closed-loop bandwidth is 25.7 MHz. The additional channels reduce the closed-loop bandwidth, even if the input signal continues to reach a gain of –2V/V.

If a current feedback amplifier is used as shown in the circuit of Figure 1, the amplifier closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. If designing a circuit with such an amplifier, the first thing to do is to select the appropriate feedback resistor, the specifications of each manufacturer, and the circuit noise gain. Then choose the appropriate RIN value. From this point, if the circuit adds channels, perhaps a small change will occur with a sharp increase in signal bandwidth and gain. If that situation occurs, back off and determine the choice of feedback resistor. For current and voltage feedback amplifiers, the noise gain is usually equal to the result of Equation 1, but reducing the feedback resistor value in the current feedback amplifier circuit increases the circuit bandwidth.

Original English:

Voltage- and current-feedback amps are almost the same

The application-circuit configurations for voltage- and current-feedback amps are generally the same, except for a few key points.

By Bonnie Baker -- EDN, 10/25/2007

Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. As such, current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. The name “current-feedback amp” carries some mystique, but, generally, the application-circuit configurations for voltage- and current-feedback amps are the same, except for a few key points.

First, the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. Lower value resistors reduce the current-feedback amp’s stability. The feedback resistor’s higher values reduce the current-feedback amp’s bandwidth. You can find the prescribed feedback-resistor value in the current-feedback amp’s product data sheet. The voltage-feedback-amp’s feedback-resistance value is more forgiving. This amplifier’s drive capability limits the resistor’s minimum value, and the overall circuit noise limits the maximum value.

Figure 1 sho ws a circuit that is appropriate for either a current- or a voltage-feedback amp. If the feedback resistance, RF, equals 2RIN, where RIN is the input resistance, the closed-loop gain of each channel is –2V/V. At first glance, it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel’s gain, or |–2V/V|. Don’t make this assumption!

If you use a voltage- or current-feedback amp with the circuit in Figure 1, the noise gain is:

where N is the number of input channels. This circuit’s bandwidth, with a voltage-feedback amp, equals the gain-bandwidth product divided by the noise gain. For instance, if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V, the circuit’s closed-loop bandwidth is 25.7 MHz. Additional channels reduce the closed-loop bandwidth, even though the input signals continue to see a gain of –2V/V.

If you use a current-feedback amp with the circuit in Figure 1, the amplifier’s closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. If you design this circuit with such an amp, you would first pick the optimum feedback resistor, per the manufacturer’s specification and the circuit’s noise gain. You would then select the appropriate value for RIN. From this point, if you add channels to the circuit, a small variation in the signal bandwidth and gain peaking in circuit may occur. If that scenario is a concern, go back and refine your feedback-resistor selection. For both current- and voltage-feedback amps, the noise gain always equals the result of Equation 1, but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth.