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RF Electronics Chapter 10: Operational Amplifiers Page 342 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. g in f o in error R V R V V i Eqn. 10.7 Since error o I s Z V ) ( , substituting for I error in the above equation after manipulation leads to: � � � � ������ � ���� �� Eqn. 10.8 The current feedback amplifier will thus have a transfer function of: � �� � �� � � ���� � Eqn. 10.9 Compared with the transfer function of the voltage feedback amplifier of: � �� � �� � ���� � Eqn. 10.10 Where g g f R R R G is the gain of an ideal amplifier Eqn. 10.11 The amplifier corner frequency is when G/A(s) = 1 or when R f /Z(s) = 1. The way G/A(s) and R f /Z(s) changes with frequency will thus result in a difference in the bandwidth of the amplifiers. Halving both R f and R g , keeps the gain the same for both amplifier configurations. That will double the bandwidth of the current feedback amplifier, but not change the bandwidth for the voltage feedback amplifier. Halving R g and keeping R f the same, will increase the gain G, as per equation 10.10 for both amplifiers. According to figure 11.2, this will reduce the bandwidth of the voltage feedback amplifier by close to half. Changing R g will not change the bandwidth of the current feedback amplifier. The feedback resistor R f can only be varied over a small range. For example, the LMH6702 has been optimised for a 237 Ω feedback resistor. Using a lower value can lead to excessive ringing in a pulse response and a higher value will limit the bandwidth. Current and Voltage Feedback Amplifier Comparison Voltage feedback (VFB) OpAmps, are normally used for teaching about OpAmps at universities and as a result, most engineers are very familiar with them. Voltage feedback OpAmps have high open loop gains, low offset voltage and low input bias currents. The +ve and –ve input circuit architectures are the same, so that input bias and noise currents for both inputs of VFB OpAmps are approximately equal. For FET input OpAmps the bias currents can be less than 1 pA. For low voltage power supplies, rail to rail inputs and outputs are required. The vast majority of rail-to-rail OpAmps are voltage feedback devices. Current feedback OpAmps have a lower open loop gain and lower input impedances. That results in less gain accuracy. The +ve and –ve input circuit architectures are different, so that the input bias and noise currents are different for the +ve and –ve input. Current feedback amplifiers are thus not very suitable for integrators. Current feedback OpAmps, are optimised for a fixed feedback resistor and that makes it more difficult for them to be used for active filters. Current feedback amplifiers have larger bandwidths, higher slew rates and generally have a lower distortion than voltage feedback amplifiers, as can be seen in the table 10.1. The vast majority of high bandwidth, high output power OpAmps, RF Electronics: Design and Simulation 342 www.cadence.com/go/awr

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