Common-Collector & Common-Drain Follower

1. The buffer role

👉 Simple analogy — a gearbox in 1:1 ratio

• The gearbox passes input speed through unchanged — Aᵥ ≈ 1, no voltage gain.
• But it lets you handle a much heavier load on the output shaft — same speed, different torque capacity.
• Same voltage, different current capacity. 👉 Trade voltage gain for current drive.

1.1 What it does

Takes the input voltage and reproduces it almost exactly at the output — $A_v \approx 1$. No voltage amplification; the output follows the input.

1.2 Why bother?

The follower provides very high $R_{in}$ and very low $R_{out}$. Its job is impedance transformation, not voltage gain.

1.3 Where it sits

Always near the output of a signal chain — between a high-impedance source (e.g. CE amp collector) and a low-impedance load (headphones, speaker, sensor input). It absorbs the heavy load without collapsing the upstream gain stage.

2. Why Aᵥ ≈ 1

2.1 The setup

Input at the base, output at the emitter, $R_E$ from emitter to ground. The collector connects directly to $V_{CC}$ — AC ground.

2.2 Apply the model

The controlled current $g_m \cdot v_{be}$ flows through $R_E$, developing $v_{out} = g_m v_{be} \cdot R_E$. But $v_{be} = v_{in} - v_{out}$, so the output feeds back and reduces the driving voltage.

2.3 Compute

Solving the loop:

Where:

• $g_m$ — transconductance, typically 40–100 mA/V at $I_C \approx 1\,\text{mA}$.
• $R_E$ — emitter resistor, typically 1–10 kΩ.
• With $g_m R_E \approx 40$ (typical), $A_v = 40/41 = 0.976$ — within 2.5% of unity.

3. High Rɪɴ, low Rₒᴜᴛ

👉 Simple analogy — a pipe reducer / faucet adapter

• The input port is a wide pipe (high Rɪɴ — easy for the source to drive).
• The output port is a narrow nozzle (low Rₒᴜᴛ — high pressure, drives small loads stiffly).
• 👉 Same flow rate (voltage), different pipe diameters (impedance).

3.1 Rɪɴ

Looking into the base, the signal sees $r_{\pi}$ in series with $(\beta+1) \cdot R_E$. For $\beta=150$, $R_E = 1\,\text{k}\Omega$: $R_{in,BJT} \approx 152\,\text{k}\Omega$. Enormous compared to a CE stage's $r_{\pi}$ alone.

3.2 Rₒᴜᴛ

Looking into the emitter, the transistor acts as a current divider — the impedance is divided by $(\beta+1)$. Typical values are a few ohms to a few tens of ohms.

3.3 The contrast

Input looks huge, output looks tiny. That is the impedance transformation — the same $(\beta+1)$ multiplier that makes $R_{in}$ huge simultaneously makes $R_{out}$ tiny.

Where:

• $r_{\pi} = \beta / g_m$ — the BJT input resistance (1–5 kΩ).
• $(\beta+1) \approx \beta$ for large β — the β-multiplier that works in both directions.

4. The Vʙᴇ drop pitfall

4.1 The catch

The emitter sits one $V_{BE} \approx 0.7\,\text{V}$ below the base. So $V_{out} = V_{in} - V_{BE}$ — a fixed DC offset, not a signal error.

4.2 Limits headroom

If $V_{in}$ approaches $V_{CC}$, the output clips approximately 0.7 V below $V_{CC}$. The top of the swing is hard-limited.

4.3 Bias matters

The input must be biased high enough that $V_{out} \geq 0$ at the negative peak. Voltage-divider bias handles this cleanly — set $V_B$ so the Q-point sits mid-swing.

5. The follower's load line

The CC follower lives on the same output characteristic family as the CE amp — but the load line and the output node are different.

5.1 Output is at the emitter

In a CC follower, $V_{out}$ is the emitter voltage, not the collector voltage. So while $V_{CE}$ swings around the load line just like in a CE amp, the output we read is $V_{out} = V_E = V_{in} - V_{BE}$ — it tracks the input directly, with no inversion.

5.2 The load line is set by $R_E$

$R_E$ (instead of $R_C$) sets the load line slope. Common $R_E$ values are 1–10 kΩ — sometimes the same as a CE's $R_C$, sometimes very different. The Q-point still sits where the load line crosses the chosen $I_B$ curve.

5.3 No clipping at saturation… same clipping at cut-off

Because output is at the emitter and $A_v \approx 1$, the follower's output rail is roughly $V_{CC} - V_{BE}$ at the high end and ground at the low end. Hit the bottom of the load line (cut-off) and the output flat-tops at ground; hit the top and the output flat-tops at $V_{CC} - V_{BE}$.

• Top of load line — $V_{CE}$ small, $V_E \approx V_{CC} - V_{CE}$ (close to $V_{CC}$). Output limited by $V_{CC}$ headroom.
• Bottom of load line — $I_C \approx 0$, $V_E \approx 0$. Output flat-tops at ground.
• Stay in the active region — the only place $A_v \approx 1$ holds cleanly.

Simple analogy — water pipe with the spout at a different height

• CE amp = spout at the top of the pipe (collector). Pressure swings hugely as flow changes — like a fountain.
• CC follower = spout partway up the pipe (emitter). Output pressure tracks the input pressure with a constant offset (the $V_{BE}$ drop) — like a faucet with a built-in pressure regulator.
• 👉 Same plumbing, different output port.

Try it in the simulator — drop $R_L$ very low and watch the follower's $V_{out}$ hold near unity-gain while the direct-drive panel collapses.

Try it in the simulator

Open the Simulate tab — drag the Rʟ slider down and watch the direct-drive panel collapse while the buffered panel holds up. That is the CC follower in action.