Voltage Buffer
– Used to transfer voltage from a high output impedance circuit to a low input impedance circuit
– Prevents the second circuit from loading the first circuit and interfering with its operation
– Ideal voltage buffer has infinite input resistance and zero output resistance
– Provides unity voltage gain, but can have considerable current gain and power gain
– Thévenin equivalent circuit of a voltage buffer is an ideal voltage source with zero Thévenin resistance
Current Buffer
– Used to transfer current from a low output impedance circuit to a high input impedance circuit
– Prevents the second circuit from loading the first circuit and interfering with its operation
– Ideal current buffer has infinite output impedance and zero input impedance
– Provides unity current gain, acting as a current follower
– Norton equivalent circuit of a current buffer is an ideal current source with infinite Norton resistance
Voltage Buffer Examples
– Op-amp implementation can create a unity gain buffer by applying negative feedback
– Op-amp voltage follower circuit has high input impedance and low output impedance
– Impedance of the circuit comes from the input and output impedances of the op-amp
– Bridging connections reduce power consumption, distortion, and interference
– Single-transistor circuits like BJT and MOSFET can also act as voltage followers
Impedance Transformation using the Bipolar Voltage Follower
– Impedance seen looking into the circuit is increased by a factor of (β + 1), where β is the transistor gain
– The impedance is further increased by the added load resistance
– Analysis uses the relation between current and voltage in the transistor
– The impedance looking into the buffer is larger than the load without the buffer
– The added impedance does not make much difference if it is much smaller than (β + 1) times the load resistance
Impedance Transformation using the MOSFET Voltage Follower
– Impedance seen looking into the circuit is infinite at low frequencies
– As frequency increases, the parasitic capacitances of the transistors come into play
– Transformed input impedance drops with frequency due to the capacitances
– MOSFET draws no current, resulting in an infinite input impedance
– MOSFET voltage follower can be used as a buffer to isolate the driver from the load Source: https://en.wikipedia.org/wiki/Buffer_amplifier
A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents (or voltages, for a current buffer) that the load may impose. The signal is 'buffered from' load currents. In other words, a buffer turns a non-ideal voltage or current source into a "more ideal" one, that is, one with a lower internal impedance. Two main types of buffer exist: voltage buffer and current buffer.
