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I've laid out two examples of a bootstrap network in two applications. A bootstrap is a deliberate controlled positive feedback in an amplifier as to increase the impedance of the bootstrapped circuit, making it appear as a constant current source as far as AC is concerned. The only requirement for a stable bootstrap is that the positive feedback has a coefficient of less than 1, otherwise oscillations may occur. The first example uses the bootstrap to increase the input impedance of the circuit by returning some of the output signal, which is in-phase with the input signal. Since the emitter follower has a voltage gain of a little less than 1, there is no danger for oscillations to occur. For that same reason though, the input impedance cannot be made infinite because the returned signal is not exactly the same voltage as the input, as to cancel the current consumption completely. The second example uses the bootstrap to increase the linearity and gain of a common emitter voltage amplifier. The collector load is essentially split in half, and the output signal is fed through a capacitor at the center tap of the split load. This way the potential difference (voltage drop), across the load remains constant (ish) and thus the current also remains constant, essentially becoming a constant current source as far as the AC signal is concerned. This increases the gain and the linearity. The alternative is to use an active constant current source, but sometimes a CCS is not applicable, and a bootstrap is needed. The two methods essentially do the same thing, but I personally prefer the CCS method due to numerous reasons, I won't be getting into now.
You can flip the switches and see the effect of the bootstrap on the input impedance on the first circuit, and the gain in the second one. The linearity measurements require a different, more complex set-up, so I've decided to skip them.
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