Buffer amplifier

As an example, consider a Thévenin source (voltage VA, series resistance RA) driving a resistor load RL.

However, if the Thévenin source drives a unity gain buffer such as that in Figure 1 (top, with unity gain), the voltage input to the amplifier is VA, and with no voltage division because the amplifier input resistance is infinite.

As an example, consider a Norton source (current IA, parallel resistance RA) driving a resistor load RL.

However, if the Norton source drives a unity gain buffer such as that in Figure 1 (bottom, with unity gain), the current input to the amplifier is IA, with no current division because the amplifier input resistance is zero.

A unity gain buffer amplifier may be constructed by applying a full series negative feedback (Fig.

Because the output impedance of the op-amp is very low, it drives the load as if it were a perfect voltage source.

All such amplifiers actually have a gain of slightly less than unity (though the loss may be small and unimportant) and add a DC offset.

The impedance is increased even more by the added rπ, but often rπ << (β + 1) RL, so the addition does not make much difference Using the small-signal circuit in Figure 5, the impedance seen looking into the circuit is no longer RL but instead is infinite (at low frequencies) because the MOSFET draws no current.

For most digital applications, an NMOS voltage follower (common drain) is the preferred configuration.

[dubious – discuss] These amplifiers have high input impedance, which means that the digital system will not need to supply a large current.

A non-linear buffer amplifier is sometimes used in digital circuits where a high current is required, perhaps for driving more gates than the normal fan-out of the logic family used, or for driving displays, or long wires, or other difficult loads.

The AC output current iout is delivered by the buffer via a large coupling capacitor to load RL.

Figure 1: Ideal voltage buffer (top) and current buffer (bottom)
Figure 2: A negative feedback amplifier
Figure 3. An op-amp –based unity gain buffer amplifier
A voltage follower boosted by a transistor ; also can be seen as the "ideal transistor" without a base-emitter forward bias voltage drop on the input signal. This is the basic circuit of linear voltage regulators
Figure 4: Top: BJT voltage follower Bottom: Small-signal, low-frequency equivalent circuit using hybrid-pi model
Figure 5: Top: MOSFET voltage follower Bottom: Small-signal, low-frequency equivalent circuit using hybrid-pi model
Figure 6: Bipolar current follower biased by current source I E and with active load I C