10
\$\begingroup\$

Op-Amp

I understand that, for an ideal opamp, Vo is bounded by Vee and Vcc (i.e.; Vee < Vo < Vcc).

But, what about the input voltage range? What is the allowed input voltage range in which opamp works normally? Can we apply Vn and Vi voltages which are below Vee or above Vcc? For single supply opamps, can we apply negative input voltages?

Does opamp being ideal, practical, rail to rail change the answer of this question?

\$\endgroup\$
10
\$\begingroup\$
  • In normal operation, V+ and V- are the same. If they are different, the op-amp is being used as a comparator.

  • An important spec in choosing an op-amp is the "input common-mode voltage" range. The voltages at the inputs must be in this range for proper operation.

  • Some op-amps are "RRIO" or rail-to-rail input/output. As you might guess, this means the input common mode range goes from the Vee rail to the Vcc rail.

  • Some op-amps which include only the Vee rail in the common-mode range are referred to as "single-supply" op-amps.

Here is the spec in the LM324 datasheet: enter image description here

\$\endgroup\$
2
\$\begingroup\$

It's important to note that some op amps can behave very strangely if either input voltage gets too close to the supply rails. If an op amp has a common-mode voltage range that extends down to 0.5 volts, for example, that would clearly imply that if both inputs are below 0.5 volts the op amp might be unable to distinguish which is higher. On some such op amps, however, an input which goes below 0.5 volts might be regarded as being higher than the other input--even if the other input is well above 0.5 volts.

\$\endgroup\$
2
\$\begingroup\$

(1) What they all said.

(2) Note that a few opamps can swing to slightly beyond supply rails - this is achieved by generating voltages within the opamp that give it a true operating voltage wider than what is supplied.

(3) Note that common mode range is allowable voltage which eitherm input MUST be within. It is not the difference between them. If either is outside the range then it is not guaranteed to work.

\$\endgroup\$
1
\$\begingroup\$

The input voltage range can vary, but has to be within the supply voltages (you may see some that can go e.g. 0.3V above/below) so if single supply then no negative voltages are allowed.
It depends on how the differential input stage is set up, some opamps use a complementary input pair (P-channel and N-channel in parallel) to achieve a rail to rail input, each pair covering part of the range.
Generally there will be information about the common mode input range in the datasheets, here is an example of an opamp without a rail to rail input range (specs are for +/-5V):

Common mode range example

\$\endgroup\$
1
\$\begingroup\$

The specification you are looking for is called "input common mode range", and is almost always limited to the supply rails. On many opamps, it's limited to somewhat less in one or both directions.

You can take a normal opamp and make one with a large input voltage range exceeding the supply limits by putting a resistor divider in front of each input. The input offset voltage also gets multiplied by the same value though, and it will cut down on common mode rejection to the extent that the dividers aren't exactly matched.

If your input signal is AC, then you can get a very large input common mode range by transformer coupling.

\$\endgroup\$
0
\$\begingroup\$

But note in a inverter the input to the resistor connected to the inverting input can easily be much larger than the op amp limit. The overall circuit function can often keep the inverting input within range.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.