Buffer Amplifier - Current Buffer

Current Buffer

Typically a current buffer amplifier is used to transfer a current from a first circuit, having a low output impedance level, to a second circuit with a high input impedance level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. In the ideal current buffer in the diagram, the input impedance is zero and the output impedance is infinite (impedance of an ideal current source is infinite). Again, other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.

For a current buffer, if the current is transferred unchanged (the current gain β_i is 1), the amplifier is again a unity gain buffer; this time known as a current follower because the output current follows or tracks the input current.

As an example, consider a Norton source (current I_A, parallel resistance R_A) driving a resistor load R_L. Because of current division (also referred to as "loading") the current delivered to the load is only I_A R_A / ( R_L + R_A ). 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 I_A, with no current division because the amplifier input resistance is zero. At the output the dependent current source delivers current β_i I_A = I_A to the load, again without current division because the output resistance of the buffer is infinite. A Norton equivalent circuit of the combined original Norton source and the buffer is an ideal current source I_A with infinite Norton resistance.