Buffer amplifier

A buffer amplifier is an amplifier with roughly unity voltage gain, very high input resistance, and very low output resistance. It does not make a signal bigger; it makes a signal robust — it isolates a weak source from a heavy load so the connection itself does not destroy the signal.

The loading problem it solves

[Background from general knowledge, not the source PDF] Any real source can be drawn as its Thévenin equivalent: an ideal voltage $v_S$ behind a source resistance $R_S$. Connect it to a load $R_L$ and the load only receives the divided voltage

$v_L=v_S\frac{R_L}{R_S+R_L}$

(a Voltage divider). If $R_S$ is comparable to or larger than $R_L$ — a high-impedance sensor driving a low-impedance stage, a long cable, a divider tap feeding the next block — most of the signal is lost across $R_S$ before it ever reaches the load. The fix is not more gain; it is impedance transformation. Insert a buffer:

• its high input resistance ($R_{in}\to \infty$) means it draws almost no current from the source, so $v_S$ appears at the buffer input essentially undivided;
• its low output resistance ($R_{out}\to 0$) means it presents a stiff voltage to $R_L$, so the load gets the full voltage with no further division.

The voltage is unchanged (gain $\approx 1$) but the impedance “chain” is broken: the source now sees infinite load, the load now sees zero source. This is exactly the Input and output resistance (amplifier) story applied deliberately.

Where it shows up

The cleanest buffer is the op-amp Voltage follower (op-amp) — infinite $R_{in}$, zero $R_{out}$, gain exactly $1$ thanks to Negative feedback. The single-transistor versions are the MOSFET Source follower (common-drain) and the BJT Emitter follower (common-collector): both have voltage gain just under unity, high input resistance, and low output resistance, which is why they always sit at the output of a multi-stage amplifier, driving the final load, or between a high-gain stage and a low-impedance line. Think of a buffer as a real circuit’s best attempt at an ideal voltage source: it copies a voltage and supplies whatever current the load demands without letting the load disturb the original.