Common Base, Common Gate & Cascode

Analog Electronics · Common Base, Common Gate & Cascode · Learn

1. The current-buffer role

Figure 1. CB (left) and CG (right) amplifiers — input at emitter/source, output at collector/drain, base/gate held at AC ground via voltage-divider bias.

👉 Simple analogy — a one-way valve in a high-pressure pipe

Low $R_{in}$ = the input port accepts whatever current is pushed in.
High $R_{out}$ = the output port maintains constant flow regardless of downstream pressure.
👉 Pass the current through, isolate the input from the output.

1.1 What it does

Aᵢ ≈ 1 (current gain near unity), Aᵥ large, low Rɪɴ (≈ 1/gₘ), high Rₒᴜᴛ (≈ rₒ). The output looks like an ideal current source.

1.2 Why “current buffer”

The CB/CG stage passes current through nearly unchanged but presents very high output impedance. A signal current pushed into the emitter/source exits at the collector/drain essentially intact.

1.3 Why CB/CG alone is rarely used

Low Rɪɴ is awkward to drive from a typical voltage source. Connecting a voltage source directly to the emitter means large input currents and potential loading.

2. The cascode pairing

Figure 2. Cascode amplifier — CE (Q1) on the bottom, CB (Q2) on top. Q2's emitter holds Q1's collector at a near-constant voltage, killing the Miller multiplication of Cμ.

👉 Simple analogy — putting a heavy lid on a boiling pot

Without the lid (single CE): the pot's lid (the collector node) swings violently with the signal — that's Miller multiplication.
With the lid (CB on top): the pot's lid is held still by the heavy weight (CB's low input Z). Steam (output current) escapes through the lid pipe.
👉 The lower stage is freed from the Miller penalty.

2.1 The setup

Stack a CE/CS (Q1) underneath a CB/CG (Q2). The output is taken at the top — the CB collector. Both transistors share the same DC current path.

2.2 Why it works

The CB stage has a very low input impedance (rₑ = 1/gₘ ≈ tens of Ω). The CE collector node therefore sees a near-AC-ground above it — voltage swing at that node is tiny.

2.3 The payoff

Miller multiplication of Cμ depends on the voltage gain at the lower stage's collector. If that voltage barely swings, Miller capacitance vanishes and bandwidth soars.

3. Frequency-response payoff

Figure 3. Bode magnitude curves for CE alone (blue) and cascode (green). Same 40 dB gain, but the cascode bandwidth is 101× wider.

3.1 CE alone

Miller effect multiplies Cμ at the input:

f_{3dB,CE} \approx \frac{1}{2\pi \cdot R_S \cdot C_{\mu} \cdot (1 + |A_v|)}

3.2 Cascode

The CB stage holds Q1's collector still — no Miller multiplication:

f_{3dB,cascode} \approx \frac{1}{2\pi \cdot R_S \cdot C_{\mu}}

3.3 Improvement ratio

The bandwidth improvement is exactly the Miller multiplier that the cascode eliminates:

\frac{f_{3dB,cascode}}{f_{3dB,CE}} = (1 + |A_v|)

Where:

Rₛ — source resistance driving the amp (typically 1–10 kΩ).
Cμ — collector-base capacitance, typically 1–10 pF.
|Aᵥ| — voltage gain magnitude (typically 50–200). For |Aᵥ| = 100, improvement = 101×.

4. Same gain, more bandwidth

The cascode has the same low-frequency voltage gain as the CE stage alone. The CB stage on top contributes Aᵥ ≈ 1 multiplicatively — it adds nothing to the gain magnitude.

A_{v,cascode} \approx -g_m \cdot (R_C \| r_{o,cascode} \| R_L)

The only change is that $r_{o,cascode}$ of the stacked pair is much larger (the CB stage boosts it), which slightly increases gain compared to a single CE — but to first order it is the same shape.

The output characteristic and load line look the same as a CE amp — output is still at the (upper) collector, $R_C$ still sets the load line slope, the Q-point still sits where the line crosses the chosen $I_B$ curve. The cascode's contribution is frequency response, not the DC story. See Topic 1 for the load-line refresher.

Cascode = CE/CS + CB/CG stack. Same gain, 100× bandwidth. The CB/CG kills Miller capacitance by holding the lower stage's collector at AC ground. Standard topology in op-amps and RF amplifiers.

Try it in the simulator

Open the Simulate tab to explore the bandwidth interactively. Slide Rₛ and Cμ and watch the cascode bandwidth stay an order of magnitude (or two) ahead of the CE-alone curve. The improvement ratio is roughly (1 + |Aᵥ|), the Miller multiplier the cascode kills.