Bootstrapping

Art of Electronics 3rd ed. Figure 2.80 Increasing the impedance of the biasing network of the emitter-follower. R1=10KΩ R2=10KΩ R3: 4.7KΩ R4: 1KΩ Without bootstrapping the input impedance would be R1||R2||βR4 but because R1||R2 << βR4 (usually 10x) the input impedance is about R1||R2. With bootstrapping R3 acts as a current source (because it is a follower, Vbe ≈ Ve) and current source has infinite impedance. The effective impedance is less than infinity, because the gain of the follower is less than unity. The bootstrapped circuit imput impedance is dominated by βR4, so it is about 100K. When the switch is open, the signal generator has an output impedance of 100KΩ. When the switch is closed, the imput impedance is 0Ω Closed switch output swing: 1.98V Opened switch output swing: 0.84V When the input signal has a 100KΩ impedance the output swing is roughly halved, so the bootstrapped circuit has an input impedance about 100K.

published 9 years ago