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.

An earlier article published in EDTN discussed the gain-bandwidth relationship for CFB and VFB amplifiers. This article will examine the basic CFB and VFB topologies to explain why VFB amplifiers have better dc specifications and why CFB amplifiers tend to have faster slew rates and better distortion performances.

An Internal Look at VFB Topology

It is a commonly-known fact that VFB amplifiers tend to have better dc specifications than CFB amplifiers. A close look at the input stage of the VFB topology will explain this fact. A typical VFB amplifier input stage is shown in Fig. 1.

Low Input Offset Voltage: A VFB input stage is often a simple differential pair, two identical bipolar transistors at the same bias current and voltage. This configuration is often called a balanced circuit because of the symmetry between the two inputs. Because of this symmetry, there will be no input offset voltage unless the devices do not match.

Matched Input Bias Currents: The inputs are the bases of the two transistors. Although the absolute base currents, or input bias currents, may vary considerably due to process variation and temperature, they will match unless, again, the devices are not identical.

Good CMRR and PSRR: When either the supply voltage or the common-mode input voltage is altered the change in the collector-to-emitter voltages is matched for both of the input transistors. Changes in the devices' bias point could affect offset but, again due to the balanced topology, the bias currents match and offset voltage is little affected. The result of this is good CMRR and PSRR.

An Internal Look at CFB Topology

The input stage of a CFB amplifier will also describe a few inherent dc traits of the CFB amplifier.

Non-zero Input Offset Voltage: The input stage of a typical CFB amplifier is illustrated in Fig. 2. It is a voltage buffer and for the offset voltage to be zero, the VBE of the NPN transistors would have to match the VBE of the PNP. Since these devices are inherently different in their construction, there is no reason why they would inherently match.

Unmatched Input Bias Currents: Bias currents in CFB amplifiers are also fundamentally mismatched as the non-inverting bias current is the difference between two base currents, where the inverting bias current depends on the errors produced in the next stage.

Advantages of the CFB Topology

One hidden advantage of current feedback amplifiers is that they usually require fewer internal gain stages than their voltage feedback counterparts. Often a current feedback amplifier merely consists of an input buffer, one gain stage and an output buffer. Having fewer stages means less delay through the open-loop circuit and this translates into higher bandwidths for the same power.

The basic CFB topology in Fig. 3 is a single-stage amplifier. The only high-impedance node in the circuit is at the input to the output buffer. VFB amplifiers usually require two or more stages for sufficient loop-gain. These additional stages add delay and yield lower, stable, bandwidths.

Distortion

The distortion of an amplifier is impacted by the open-loop distortion of the amplifier and the overall speed of the closed-loop circuit. The amount of open-loop distortion contributed by a CFB amplifier is actually low because of the basic symmetry of the topology. Fig. 3 illustrates a typical CFB topology and for every NPN transistor, there is a complementary PNP transistor. Speed is the other main contributor to distortion and in many gain configurations a CFB amplifier has a greater bandwidth than its VFB counterpart. So at a specified frequency the faster part has greater loop-gain and therefore lower distortion.

Slew Rate

Slew-rate performance is also enhanced by the CFB topology. In the typical CFB topology of Fig. 3 the slew rate is determined by the rate at which the second two transistors can charge the compensation capacitors, Cc. The current that can be sourced by these transistors is dynamic and it is not limited to any fixed value as is often the case in VFB topologies. With a step input, or overload condition, the current flowing through the transistors is increased and the overdriven condition is quickly removed. To the first order, there is no slew rate limit in this architecture. Some VFB amplifiers have input structures similar to CFB amplifiers in order to take advantage of the higher slew-rate possibilities. The combination of higher bandwidths and slew rate allows CFB devices to have respectable distortion performance while doing so at a lower power.

The basic current feedback amplifier has no fundamental slew-rate limit. Limits only come about by parasitic transistor capacitances and many strides have been made to reduce even their effects.

The availability of high-speed operational amplifiers in both CFB and VFB topologies allows design engineers to select the best amplifier to fit his/her needs. A CFB amplifier complements an application that requires high slew rates, low distortion, or the ability to set gain and bandwidth independently. A VFB amplifier complements an application where low offset voltage or low-noise specifications are required.