CHAPTER
7
Input and VAS Circuits
T
he amplifier that was evolved in Chapter 3 served as a good platform for amplifier design understanding, but it did not include significant sophistication of the input stage (IPS) and voltage amplifier stage (VAS) circuits. Rather, it started with the most basic IPS-VAS and evolved it in a linear way to achieve much-improved performance. Although the end result was quite good, there are many ways to skin a cat and achieve further improved performance. Moreover, the analysis of the IPS-VAS was fairly superficial, for example, there was little discussion of noise.
7.1 Single-Ended IPS-VAS The single-ended IPS-VAS was discussed at length in Chapter 3 where a basic amplifier was evolved to a high-performance amplifier. Most of the evolution in the design took place in the IPS-VAS. It is referred to as single ended because the VAS is single ended with a current source load. Later in this chapter we will focus on designs that include a push-pull VAS for improved performance. The IPS-VAS shown in Figure 7.1 is unlike the simple IPS-VAS that was used as a starting point in Chapter 3. It is provided with ±45-V rails that correspond to an amplifier capable of delivering about 100 W into an 8-W load. This IPS-VAS includes emitter degeneration and is arranged with output stage predrivers and drivers as if a Triple EF was being used for the output stage. The output stage is not present, and the feedback is taken from a center tap on the driver emitter bias resistor. This allows distortion of the IPS-VAS to be evaluated in the absence of the distortion of an output stage. The pair of 234-Ω emitter degeneration resistors implements 10:1 degeneration of the input differential pair by increasing the total emitter-to-emitter resistance RLTP from 52 Ω to 520 Ω. This reduces its transconductance by a factor of 10. Recall the relationship described in Chapter 2 for Miller compensation:
CMiller = 1/(2 p fc RLTP Acl )
where Acl is the closed-loop gain, RLTP is the total emitter-to-emitter LTP resistance including re’, and fc is the desired gain crossover frequency for the negative feedback loop. Setting fc to 500 kHz and closed-loop gain to 20, we have
C1 = CMiller = 0.159/(500 kHz * 520 Ω * 20) = 30.6 pF
By this calculation C1 must be about 30 pF. Notice that ±1 mA is available from the input stage to charge and discharge C1. This results in an achievable slew rate of 1 mA/30 pF = 33 V/µs. This is a respectable value of slew rate for an audio power amplifier of modest power level.
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