Question 11.14: Consider a cascode amplifier with transistor and the circuit...

Here, R^{'}_{s} = R_{s} || r_{\pi} = 300 || 2000 = 260  \Omega .

For cascode amplifier

We know that                       \omega _{H}=\frac{1}{R^{'}_{s}(C_{\pi }+2C_{\mu })}

Here,                     C_{\pi} + 2C_{\mu} = 19.5 + 2 × 0.5 = 20.5  pF

Therefore,                              f_{H} =\frac{\omega _{H}}{2\pi } =\frac{1}{2\pi \times R^{'}_{s}(C_{\pi }+2C_{\mu })}

=\frac{1}{2\pi \times 260\times (20.5\times 10^{-12})} \approx 30  MHz

The mid-band gain,

G=\frac{R_{C}\beta }{r_{\pi } }=\frac{1.5\times 10^{3} \times 100}{2\times 10^{3} }=75

For CE amplifier

Considering the CE amplifier with same transistor and gain, G = 75

C_{T} = C_{\pi} + GC_{\mu} = 19.5 × 10^{–12} + 75 × 0.5 × 10^{–12} = 57  pF

f_{H} =\frac{1}{2\pi R^{'}_{s}C_{T }}=\frac{1}{2\pi \times 0.26\times 10^{3}\times 57\times 10^{-12}} =10.7  MHz

It is inferred that with the same mid-band gain, the cascade configuration has a bandwidth three times that of CE configuration.