Consider a cascode amplifier with transistor and the circuit parameters are rπ = 2 kΩ, gm = 0.05 Ω, = 100, Cπ = 19.5 pF, Cμ = 0.5 pF, Rs = 300 Ω and RC = 1.5 kΩ. Determine fH and mid-band gain G of cascode amplifier and CE amplifier.

Question

Consider a cascode amplifier with transistor and the circuit parameters are [latex]r_{\pi}[/latex] = 2 kΩ , [latex]g_{m}[/latex] = 0.05Ω , = 100, [latex]C_{\pi}[/latex] = 19.5 pF, [latex]C_{\mu}[/latex] = 0.5 pF, [latex]R_{s}[/latex] = 300Ω and [latex]R_{C}[/latex] = 1.5 kΩ . Determine [latex]f_{H}[/latex] and mid-band gain G of cascode amplifier and CE amplifier.

Accepted Answer

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

For cascode amplifier

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

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

Therefore, [latex]f_{H} =\frac{\omega _{H}}{2\pi } =\frac{1}{2\pi imes R^{'}_{s}(C_{\pi }+2C_{\mu })} [/latex]

[latex]=\frac{1}{2\pi imes 260 imes (20.5 imes 10^{-12})} \approx 30 MHz [/latex]

The mid-band gain,

[latex]G=\frac{R_{C}\beta }{r_{\pi } }=\frac{1.5 imes 10^{3} imes 100}{2 imes 10^{3} }=75[/latex]

For CE amplifier

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

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

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

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

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

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