Opamp:Buffer
Use this circuit when you have a signal of high impedance ( can supply only a little current ) that you want to connect to another circuit that draws a significant current ( up to about 10 ma for the typical op amp. ). For example if you wish to measure the output of a voltage divider with a 0 to 1 ma meter a unity gain buffer might be just what you need. This circuit is also know as a voltage follower.
The unity gain buffer has an output voltage just the same as the input voltage. The advantage is that the input circuit does not “feel” the output. That is the input acts pretty much like a very large resistor ( many mega ohms or more ) connected to ground, and the output supplies whatever current ( up to about 10 ma ) is necessary to maintain the output voltage. Here is the circuit:
Circuit:
Where
- INPUT the input signal you wish to buffer
- RIN the input resistor, often 0 ohms.
- OPAMPA Any general purpose op amp, often connected to + and - power supplies
- RFB the feed back resistor
- OUTPUT the output
Discussion: The values of RIN and RFB are not very critical and are normally 0 ohms, just a straight connection. The op amp here is a quad or 4 op amp part, we are using just one section of it. Power needs to be supplied to pin 8 and 4 in the usual way for op amps. This circuit uses very large feedback (unity) and for this reason has poor stability margins. This may cause the output to go into oscillations when connected to certain loads (typically capacitive). Check the datasheet of individual opamps for details and remedies.
