MT-034: Current Feedback (CFB) Operational Amplifiers

Introduction

This tutorial provides a detailed introduction to two fundamental operational amplifier topologies: voltage feedback (VFB) and current feedback (CFB), and explains their differences. Figure 1 shows the basic voltage feedback operational amplifier and its gain formula.

It is important to note that the error signal generated by the feedback network and the finite open-loop gain A(s) is actually a small voltage v.

Basic Principles of Current Feedback Amplifiers

Figure 2 illustrates the basic current feedback amplifier topology. Note that this model uses a unity-gain buffer to connect the non-inverting input to the inverting input. Ideally, the output impedance of this buffer is zero (R_o = 0), and the error signal is the small current i flowing into the inverting input. This error current is mirrored to the high-impedance T(s), and the voltage generated across T(s) is equal to T(s) * i. (The magnitude of T(s) is typically referred to as the open-loop transconductance gain.)

Subsequently, this voltage is buffered and connected to the amplifier output. If R_o is assumed to be zero, the expression for the closed-loop gain V_OUT/V_IN, expressed in terms of the R1-R2 feedback network and the open-loop transconductance gain T(s), can be easily derived. The formula can also be derived when R_o is finite. Figure 3 shows both expressions.

It should be noted at this point that current feedback operational amplifiers are often referred to as transconductance amplifiers because the open-loop transfer function is essentially an impedance, as mentioned earlier. However, the term transconductance amplifier is also frequently used in many common circuits, such as current-to-voltage (I/V) converters, and both CFB and VFB operational amplifiers can be used for I/V converters. Therefore, care should be taken when the term transconductance is encountered in a specific application. The term current feedback operational amplifier, however, is rarely confused, so it is best to use this term when referring to operational amplifier topologies.

Several important characteristics of CFB amplifiers can be derived from this simple model:

• Unlike VFB operational amplifiers, CFB operational amplifiers do not have balanced inputs. Instead, their non-inverting input is high impedance, and their inverting input is low impedance.
• The open-loop gain of a CFB amplifier is measured in Ω (transconductance gain), not in V/V as for VFB operational amplifiers.
• When the feedback resistor R2 is fixed, the closed-loop gain of a CFB amplifier can be changed by altering R1, without significantly affecting the closed-loop bandwidth. This can be seen by examining the simplified formula in Figure 3. The denominator determines the overall frequency response. If R2 remains unchanged, R1 in the numerator can be changed (thereby changing the gain) without affecting the denominator, thus keeping the bandwidth relatively stable.

CFB Amplifier Frequency Response

If a resistor with a value lower than the recommended value is used, the phase margin will decrease, and the amplifier may become unstable.

When controlling the gain of a CFB amplifier application, the correct feedback resistor (R2) must be selected for the device, and then the lower resistor (R1) must be chosen to produce the desired closed-loop gain. The relationship between gain and resistors R2 and R1 is the same as in VFB amplifiers.

The optimal feedback resistor may differ under various operating conditions. For example, due to variations in parasitic effects, the optimal feedback resistor value can change for different package types. Figure 5 shows the optimal feedback resistors for the AD8001 operational amplifier in PDIP, SOIC, and SOT-23 packages at different gains.

Capacitors should not be placed in the feedback loop of a CFB amplifier. If a capacitor is used in the feedback loop, it will reduce the high-frequency feedback impedance, potentially causing the operational amplifier to oscillate. Stray capacitance at the inverting input can cause a similar effect, so grounding around the inverting input should be minimized to reduce stray capacitance.

Comparison of CFB and VFB Amplifiers

A common mistake when using current feedback operational amplifiers is to connect the inverting input directly to the output, attempting to create a unity-gain voltage follower (buffer). This circuit will oscillate because the equivalent feedback resistance is zero. As long as the recommended feedback resistor value is used, connecting the inverting input to the output will stabilize the follower circuit.

Another difference between VFB and CFB amplifiers is that the inverting input impedance of a CFB amplifier is low (typically 50 Ω to 100 Ω), while the non-inverting input impedance is high (typically several hundred kΩ). Therefore, CFB amplifiers have unbalanced inputs, whereas VFB amplifiers have balanced inputs.

The CFB topology also enhances slew rate performance. The current available to charge and discharge internal compensation capacitors can be supplied as needed. Unlike the typical VFB topology, it is not necessary to limit this to a fixed value. For a step input, the current will increase as needed until the feedback loop stabilizes. Basic current feedback amplifiers have no theoretical slew rate limit. The only limitations are related to internal parasitic capacitances, which have been reduced through various means.

CFB devices combine high bandwidth and high slew rate with good distortion performance and low power consumption. The distortion of an amplifier is affected by the open-loop distortion of the amplifier and the loop gain of the closed-loop circuit. Because the internal topology is fundamentally symmetrical, the open-loop distortion generated by the CFB amplifier is relatively small. High bandwidth is another major contributor to low distortion. In most configurations, CFB amplifiers have higher bandwidth than their VFB counterparts. Therefore, at a given signal frequency, they have higher loop gain and thus lower distortion. However, some voltage feedback structures (often referred to as "quad-core" or "H-bridge") are designed using similar processes and offer performance levels close to CFB, also providing "current on demand" (References 2, Chapter 1, Sections 1-6).

Summary: Current Feedback (CFB) vs. Voltage Feedback (VFB)

Current feedback and voltage feedback have different application advantages. In many applications, the distinction between CFB and VFB is not significant. Today, CFB and VFB amplifiers offer comparable performance, but the two topologies still have their unique advantages. Voltage feedback allows free selection of feedback resistors (or networks), but sacrifices bandwidth for gain. Current feedback can maintain high bandwidth over a wide range of gains but limits the choice of feedback resistors.

In summary, VFB amplifiers have the following characteristics:

• Lower noise
• Better DC performance
• Freedom in selecting feedback components

CFB amplifiers have the following characteristics:

• Faster slew rate
• Lower distortion
• Limited choice of feedback components

References

1. Hank Zumbahlen, Basic Linear Design, Analog Devices, 2006, ISBN: 0-915550-28-1. Also available as Linear Circuit Design Handbook, Elsevier-Newnes, 2008, ISBN-10: 0750687037, ISBN-13: 978-0750687034. Chapter 1.
2. Walter G. Jung, Op Amp Applications, Analog Devices, 2002, ISBN 0-916550-26-5, Also available as Op Amp Applications Handbook, Elsevier/Newnes, 2005, ISBN 0-7506-7844-5. Chapter 1.

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