Feature: Electronic design
Figure 5: Biasing the complementary folded cascodes with active current sources increases the output impedance of the amplifier by more than seven times at low frequencies compared with resistive biasing
Figure 6: Both amplifiers of Figures 1 and 4, as well as all the rest of the circuits discussed in article, were compensated to give the same unity major loop gain frequency
the disadvantage that the transconductance of a MOSFET is typically more than an order of magnitude smaller than that of a bipolar junction transistor (BJT), which means that the Class-A MOSFET source follower is significantly less linear than the equivalent Class-A BJT emitter follower. Alternatively, the loading on the cascode TIS may be
significantly reduced by isolating it from the output stage with a 48 December 2021/January 2022
www.electronicsworld.co.uk
cascade of two pairs of complementary emitter-followers (Figure 12). Tis arrangement has the advantage that the first pair of complementary drivers, Q12 and Q13, operate in Class A, even when the output stage is biased to operate in Class B, provided their emitter-coupling resistor R18 is made sufficiently small. Tis improves the isolation of the cascode TIS from the non-linear loading of the output stage.
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