Class G and H amplifiers are variations on the standard class AB. They have additional supply rails that kick in when output signal peaks would otherwise exceed the maximum voltage available from the class AB amplifier’s single voltage rail.
Class G amps feature several power rails at discrete voltage steps and switch between them as necessary. Instead of providing multiple rails, class H amps modulate the voltage on the supply rails in order to track the input signal.
Class G and H amplifiers tend to be used in audio applications. However, a related but almost forgotten alternative called the Doherty amplifier has been revived for cell-phone applications. (William Doherty was an early Bell Labs researcher.)
A Doherty amplifier comprises a class B “carrier” stage in parallel with a class C “peaking” stage. In the input, half the input signal drives one, half the other. On the output, the signals are summed. Somewhat like a class G or H amp, the class B amp carries the ball most of the time, but the class C amp cuts in on high signal peaks. The benefit of the Doherty is an increase in efficiency, relative to a pure class B.
VOLTAGE AND CURRENT FEEDBACK Most low-power amplification is accomplished using monolithic amplifier blocks, rather than discrete transistors, and gain is set by the ratio of feedback to input resistance. These can be general-purpose operational amplifiers or specialized amps tailored to the application.
These amplifiers are partly defined by the way feedback is accomplished: voltage feedback (VFB) or current feedback (CFB). Each has its tradeoffs. One significant difference lies in the presence or absence of the gain-bandwidth product characteristic.
In practical VFB amps, open-loop gain is large at dc. But above a certain frequency, it rolls off at 6 dB/octave. At some frequency, the non-inverting open-loop gain is equal to the closed-loop gain nominally set by the ratio of feedback to input resistance. At that point, the actual gain of the closed-loop configuration is 0.707 times its dc value.
This frequency is designated the amplifier’s -3-dB bandwidth. In a VFB, the product of closed-loop gain and –3-dB bandwidth is called the gain-bandwidth product (GBWP). For a VFB amp, this is a constant for a certain range of frequencies (Fig. 3a). Designers must always trade off gain for bandwidth, or vice versa.
This isn’t true for CFB amps. There, closed-loop gain is again based on external component values, but it is largely independent of frequency (Fig. 3b). On the negative side, however, CFB datasheets limit the designer’s selection of feedback resistors.
In contrast, with a VFB op amp, the circuit designer has greater freedom in choosing the value of the feedback resistor (although higher resistance values may limit stability). VFBs also offer lower noise and better dc performance than CFB amps.
Generally, CFB amps are chosen when slew rate and exceptional low distortion are needed. VFB amps excel for dc applications, for applications requiring low input bias current or high input impedance, and where rail-to-rail performance is critical.
AMPLIFIER SPECS Voltage-feedback op-amp datasheets specify five different gains: open-loop gain (AVOL), closed-loop gain, signal gain, noise gain, and loop gain. Without negative feedback, AVOL may be 160 dB or more. When an amplifier circuit’s feedback loop is closed, it exhibits less gain.
Loop gain is the difference between the open-loop and closed-loop gains or the total gain through the amplifier and back to the input via the feedback network. It comprises signal gain and noise gain. Signal gain is the gain experienced by an input signal. Noise gain reflects the input offset voltage and voltage noise of the op amp at the output. If both inputs of a differential-input amp are at 0 V, the output also should be at 0 V.
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