OA-30| Application Note

Current vs. Voltage Feedback Amplifiers

Current vs. Voltage Feedback Amplifiers
One question continuously troubles the analog design engineer: 'Which amplifier topology is better for my application, current feedback or voltage feedback?' In most applications, the differences between current feedback (CFB) and voltage feedback (VFB) are not apparent. Today's CFB and VFB amplifiers have comparable performance, but there are certain unique advantages associated with each topology. In general, VFB amplifiers offer: Lower Noise Better DC Performance

National Semiconductor OA-30 Debbie Brandenburg January 1998

Closed loop characteristics Open loop characteristics Input stage differences and advantages Once these aspects are examined, it will become apparent why VFB amplifiers have better DC specifications and why CFB amplifiers have higher bandwidths for the same power and better linear phase performance over wider bands. Finally, an internal look at the CFB amplifier will explain why distortion and slew rate are enhanced by its topology.

Feedback Freedom Aside from the well-known attribute of CFB amplifiers, gain-bandwidth independence, CFB amplifiers also tend to offer: Faster Slew Rates Lower Distortion Feedback Restrictions With these common attributes known, the design engineer may still ask: 'Why?' This article will examine the basics of the CFB amplifier in comparison with the VFB amplifier. The following aspects of each topology will be examined:

Closed Loop Characteristics
The basic amplifier design schematics and their equations hold true for both amplifier topologies. Figure 1 shows the basic circuit topologies and transfer functions for inverting and non-inverting gain configurations. These hold true for both CFB and VFB amplifiers. One point to remember is that the value of the feedback resistor is limited for CFB amplifiers. The CFB amplifier data sheet will provide the recommended Rf value.

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FIGURE 1. Basic Inverting and Non-Inverting Gain Topologies Hold True for CFB and VFB Amplifiers These transfer functions assume ideal conditions. Under ideal conditions, the open loop gain A(s) of a VFB amplifier and the open loop transimpedance gain Z(s) of a CFB amplifier are infinite. Therefore, the ideal transfer function, for the non-inverting topology, is generated as follows:

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The output is equal to the input multiplied by the gain, G.

VFB Open Loop Characteristics
The fundamental differences between VFB and CFB amplifiers begin to show when comparing their open-loop characteristics. Figure 2 illustrates the open-loop characteristics of a VFB amplifier.

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2002 National Semiconductor Corporation

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