Op Amp CircuitPublished on 24/2/2012 & updated on 5/9/2018
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Below, you can see a pin diagram of a typical Op amp.
An ideal Op amp has the following characteristics:
- An infinite open loop voltage gain.
- Infinite input impedance.
- Zero output impedance.
- Infinite common mode rejection ratio.
- Infinite bandwidth.
- Zero offset voltage.
Pin Diagram of an Op Amp ICThe op amp IC we are going to discuss about here is IC 741. It is an 8 pin IC. The pin configuration of IC 741 is given below PIN 1 - Offset Null PIN 2 - Inverting input PIN 3 - non- inverting input PIN 4 - negative voltage supply PIN 5 - offset null PIN 6 - output PIN 7 - positive voltage supply PIN 8 - not connected
The internal circuit diagram of an op amp IC is given below: As you can see above, the non-inverting terminal and offset null is located at the left. The different terminals are marked in different colours. Op amp is a collection of transistors and resistances as you can see from the picture. Op amp is used for different purposes. The 741 IC can be used asIC being used as a comparator Comparator as the name suggests compares two things. In this case, it compares two analogues signals. There will be one voltage signal which will be used as a reference and another signal which is to be compared. Op amp will then determine which of the two signal has a greater magnitude.
AC equivalent Circuit of Op ampThe equivalent circuit of an op amp is the circuit where the op amp parameters are represented in terms of physical components. Such a representation is very helpful for analysis purposes. The equivalent circuit of the op amp is given below Circuit parameters like the internal resistance, output resistance, voltage gain are all represented by circuit components like Rin, Rout etc. V1 - non-inverting input voltage with respect to ground; V2 - inverting input voltage with respect to ground; Vee - negative supply voltage; Vcc - positive supply voltage; Vout - AOLVd = AOL(v1-v2); Vd - difference between inverting voltage and non inverting voltage; Ri - internal resistance of Op amp; R0 - output resistance of op amp; AOL - open loop voltage gain.
The output voltage as you know is directly proportional to the difference between the input voltage. The voltage AOLVd is the Thevenin’s equivalent voltage source whereas Ro is the Thevenin equivalent resistance as seen from the output terminal. This is the AC equivalent circuit of an Op amp. This is very useful for analysing various op amp circuits.
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