A Simple Buffer Circuit


A buffer amplifier, or a buffer, is a circuit that provides an electrical impedance transformation from input to output. Input of the buffer has hight inpedance to prevent loading of the signal source. The output of a buffer has a low impedance. The low output impedance means that OPAMP can supply enough current to drive a relatively low impedance circuit. Two main types of buffer exist: the voltage buffer and the current buffer.
Voltage buffer transfer function is very simple: VOUT = VIN.

A voltage buffer with an input voltage divider:

Image below is a schematics of a buffer with 1/2 voltage divider. The voltage divider is designed with two resistors connected in series. The shown below voltage divider will half voltage applied to the input. This is needed because OPAMP runs from 3.3V supply and 4V input will oversaturate inputs.

Increasing input voltage to 4 Volt peak to peak, the Divided Input Signal is at 2 Volt peak to peak.

Since OPAMP supply voltage is only 3.3V, input voltages above ~4V (above ~2V at the non-inverting input of the buffer) will be clipped at the output - the green waveform has a flat top.

Increasing input voltage above 4.5V makes output waveform clipping more prominent - the green waveform has wider flat top.

There are a couple of different ways to solve this problem. V1 the 3.3V voltage supply can be selected to be of a higher voltage and/or the input divider can be changed to divide the input voltage even more.

Do not exceed the maximum supply voltage of the operational amplifier.

If you need to be able to connect the output of an OPAMP to a heavier load (when more current is needed to drive the load) a simple BJT comes handy for that purpose.

The input is applied to the noninverting op-amp terminal, and the output is connected directly to the base of the BJT. The op-amp and the BJT in this circuit use the same positive supply. The load is 100 Ohm and the maximum current flowing through the load can be as hign as 3.3V/100 Ohm = 0.033A = 33mA. 33 mA actually is not very heavy load, but an OPAMP may not be able to drive this load directly. A BJT in this case comes to the rescue.
You may notice that the output waveform is clipped at a lower point. The reason is that a BJT requires at least 0.6 - 0.7V to stay on and since the input voltage drops down to zero the output is clipped. This problem can be fixed with input biasing, resistor R4 shown below.

The buffer amplifier and BJT can be powered from separate power supplies as shown below.

In the following diagram R4 is changed from a 5.1M to a 10M resistor. See how the output is changed.

The Ideal Operetional Amplifier

The name Ideal Op Amp is applied to this and similar analysis because the salient parameters of the op amp are assumed to be perfect. There is no such thing as an ideal op amp, but present day op amps come so close to ideal that Ideal Op Amp analysis approaches actual analysis. Op amps depart from the ideal in two ways. First, dc parameters such as input offset voltage are large enough to cause departure from the ideal. The ideal assumes that input offset voltage is zero. Second, ac parameters such as gain are a function of frequency, so they go from large values at dc to small values at high frequencies.

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