Ask the Doctors: Servos
By Dr. Dave Berners

Q: What is a servo?

A: Generically, the word servo is short for servomechanism, which refers to any self-regulating feedback system. In the context of audio circuits, a servo is usually an active element used in feedback in order to null or reduce an amplifier's offset voltage.

"Servos provide a sophisticated way to reduce offset voltages in audio circuits"

Consider Fig. 1. On the left is the schematic for a differential operational amplifier (op-amp). On the right is the symbol usually used to represent this type of circuit. Ideally, the op-amp will produce an output voltage that varies linearly in proportion to the difference between the voltages present at the two inputs; that is,

Output = γ * (A - B)

where γ is the gain of the op-amp. Typical gain values for generic integrated op-amps are around 100,000, while discrete op-amps used for audio often have gain values between 100 and 10,000.

For the ideal op-amp, if voltages at points A and B are zero, the output will also rest at zero volts, in accordance with the equation given above. However, each op-amp in reality will have associated with it an offset voltage, such that

Output = γ * (A - B + η)

Here, η is the offset voltage, and reflects the fact that when inputs A and B are at zero volts, the output voltage will not be exactly equal to zero. Offset voltages arise chiefly from component-to-component variations within the op-amp, but many op-amps are designed without any attempt to reduce offset voltages. For op-amps with offset voltages, if no correction is made, a DC offset will appear on the output when the inputs A and B are held at ground.

When op-amps with large offset voltages are used, many points within a circuit may exhibit DC offsets. In audio circuits, this can lead to the following problems (among others):

• Signals biased away from ground (zero volts DC) may have reduced headroom and dynamic range.
• Switches and potentiometers connected to points biased away from ground can lead to excessive crackling and popping when controls are adjusted.
• When driving transformers with magnetic cores, inputs biased away from ground can reduce the number of volt-seconds available before saturation. This may increase transformer distortion, or may require a larger transformer.
• For some classes of amplifier, power dissipation may be increased with signals biased away from ground.

For all of these reasons, it is desirable to reduce DC offsets as much as possible at certain points within audio circuits.

Figure 2 shows the simplest method of eliminating a DC offset at a given point in a circuit (in this case, the output of the op-amp). This is a highpass filter, which passively removes the DC component of the signal.

The method used in Fig. 2 is extremely effective for removing DC, but has a few drawbacks.

First, the frequency at which the highpass filter starts removing energy depends upon the product RC of the resistance and capacitance used in the circuit. If the output of the op-amp is to have a low impedance across the audio band, it is necessary to choose a low value for the resistance used in the filter. This will necessitate using a very large capacitance in order to maintain a flat frequency response at the low end. Large capacitors have the disadvantages of taking up space and being expensive. Also, electrolytic capacitors can contribute distortion up to 0.01% at frequencies near or below the highpass cutoff frequency. The designer or user may object to this figure.

The second problem with the filter of Fig. 2 is that it is difficult to use the filtered signal as a source for negative feedback. If the gain of the op-amp is to be adjusted using negative feedback, the feedback point must be the input to the highpass filter. This means that the filter of Fig. 2 will not prevent a DC offset from appearing at the inverting input to the op-amp. With an offset appearing at the op-amp input, adjusting the gain with a potentiometer will likely produce crackling and popping.

Fig. 3 shows the use of a servo to reduce the offset voltage of an op-amp.

Figure 3. Servo and servo input-output plots

The servo monitors the output of the forward op-amp and produces a signal which, when summed into the input of the forward op-amp, reduces or minimizes its offset voltage. The servo is connected in feedback, and functions in this case as an inverting integrator. The left half of Fig. 3 shows the connection of the servo, and the right half shows the input-output relationship for the servo. As you can see, a constant offset voltage applied to the input of the servo results in an output which integrates the offset, producing a voltage which grows over time. Since this output has opposite sense when compared to the input, if the servo output is added to the noninverting ("A") input of the forward op-amp, the offset voltage will be reduced.

As compared to the filter of Fig. 2, this circuit will have the same transfer function (gain and phase) provided that the RC product for the servo is adjusted by the factor γ/2 versus the passive filter, and assuming the servo op-amp to be ideal. As long as the gain γ of the forward op-amp is not too big, this can result in a much smaller capacitor as compared to the passive filter, since a much bigger R can be chosen with the servo. For Fig. 3, the constraints on R are that it must be higher than the output impedance of the forward op-amp (which is very low), and lower than the input impedance of the servo op-amp (which is very high). The ability to use a smaller capacitor can reduce the footprint of the circuit. Also, if desired, the cutoff frequency of the servo can be placed at a lower frequency than would be practical using the filter of Fig. 2.

With the arrangement of Fig. 3, there is a residual DC offset because the servo op-amp has its own DC offset. However, the DC offset at the output of the circuit will be equal to the input offset voltage of the servo. Typically, this provides a marked decrease in offset voltage. The servo op-amp does not need to have exceptional bandwidth, slew-rate, input impedance or output impedance, so an op-amp with a very small offset voltage can usually be chosen.

It is possible that distortion arising within the servo op-amp could be transmitted to the audio path. However, at audio frequencies the gain of the servo is very small, which minimizes the amount of this distortion.

The circuit of Fig. 3 has the further advantage that the forward op-amp's inverting input B can be held to ground. Therefore, if the gain of the op-amp is to be controlled using feedback from the output to this point, a potentiometer can be connected here without worries of crackling or other artifacts when the gain is changed.

Servos provide a sophisticated way to reduce offset voltages in audio circuits, and do not require extremely large capacitances to do so.

Reference
Audio Power Amplifier Design Handbook by Douglas Self. Newnes, 2002.