Amplifiers - eclubiitk/EClub-Handbook GitHub Wiki
An operational amplifier is basically a high gain,direct coupled amplifier. It has two inputs and an one output, but generally it is operated in single ended input-single ended output mode. The differential amplifier forms the first stage of an op-amp. Opamp can perform several arithmetic operations like addition, subtraction, differentiation, integration, comparison, analog to digital conversion etc. Opamp are used is analog computers.
Figure 1.1: schematic symbol of Op-amp
IC 741 is widely used Op-amp. In this IC when input is zero, the output can be adjusted to zero by varying the 10K potentiometer between offset null terminal.
Valid only when the OA is under negative feedback
Due to the large open loop gain of the OA, the difference between the voltages at the inverting and non-inverting terminals is vanishingly small. Thus, if the non-inverting terminal is grounded (i.e. Real Ground, ), then the inverting terminal is also assumed to be at ground potential , even though actually it may not be so. This terminal is Known as Virtual Ground (to distinguish it from actual ground).
The assumption that V+=V- as well as i1=i2 is known as the Summing Point Constraint. These two assumptions make the analysis of OA circuits absolutely trivial Extending this argument, we can say that for OAs under negative feedback, the voltages at the inverting and non-inverting terminals always track each other, such that their difference is zero (almost!)
Caution:- These assumptions are applicable only for OAs under negative feedback, never apply these assumptions when the OA is under positive feedback.
Here the output of the Op-Amp is connected to its Inverting ( – ) input, thus the output is fed back to the input so as to reach an equilibrium. Thus the input signal at the Non Inverting (+) input will be reflected at the output. The Op-amp with the negative feedback will drive its output to level necessary and hence the voltage difference between its inverting and non inverting inputs will be almost zero.
Here the output voltage is fed back to the Non inverting (+) input. The input signal is fed to the Inverting input. In positive feedback design, if the Inverting input is connected to ground, then the output voltage from the Op-amp will depends on the magnitude and polarity of voltage at the Non inverting input. When the input voltage is positive, then the output of the Op-Amp will be positive and this positive voltage will be fed to the Non inverting input resulting in a full positive output. If the input voltage is negative, then the condition will be reversed.
Figure 1.2: Block diagram of Opamp
An opamp has two input terminals:
- Non-inverting input: output signal is in phase with input signal, denoted by (+).
- Inverting input: output signal is out of phase with input signal, denoted by (-).
In this configuration, the output is fed back to the negative or inverting input through a resistor (R2). The input signal is applied to this inverting pin through a resistor (R1).
The positive pin is connected to ground.
Figure 1.3: Inverting Opamp
This is evident in the special case where R1 and R2 are equal. This configuration allows for the production of a signal that is complementary to the input, as the output is exactly the opposite of the input signal.
Due to the negative sign, the output and input signals are out of phase. If both signals must be in phase, a non-inverting amplifier is used.
This configuration is very similar to the inverting operation amplifier. For the non-inverting one, the input voltage is directly to the applied to the non-inverting pin and the end of feedback loop is connected to ground.
Figure 1.4: Non-inverting Opamp
These configurations allow amplification of one signal. It’s possible to amplify several signals by using summing amplifiers.
To add 2 voltages, only 2 resistors can be added on the positive pin to the non-inverting operational amplifier circuit.
Figure 1.5: Non-Inverting Summing Amplifier
It is worth noticing that adding several voltages is not a very flexible solution. Indeed, if a 3rd voltage is added with exactly the same resistances, the formula would be Vs = 2/3 (V1 + V2 + V3).
The resistors would need to be changed to get Vs = V1 + V2 + V3, or a 2nd option is to use an inverting summer amplifier.
By adding resistors in parallel on the inverting input pin of the inverting operation amplifier circuit, all the voltages are summed.
Figure 1.6: Inverting Summing Amplifier
Unlike the non-inverting summing amplifier, any number of voltages can be added without changing resistor values.
The inverting operational amplifier amplified a voltage that was applied on the inverting pin, and the output voltage was out of phase. The non-inverting pin is connected to ground with this configuration.
If the above circuit is modified by applying a voltage through a voltage divider on the non-inverting, we end up with a differential amplifier as shown below.
Figure 1.7: Differential Amplifier
A square wave is very easy to generate, by just toggling a GPIO of a microcontroller for example. If a circuit needs a triangle waveform, a good way to do it is just integrating the square wave signal. With an Operation Amplifier, a capacitor on the inverting feedback path, and a resistor on the input inverting pin as shown below, the input signal is integrated.
Figure 1.8: Integrator Amplifier
Be aware that a resistor is often connected in parallel to the capacitor for saturation issues. Indeed, if the input signal is a very low frequency sine wave, the capacitor acts like an open circuit and blocks feedback voltage. The amplifier is then like a normal open-loop amplifier that has very high open-loop gain, and the amplifier is saturated. Thanks to a resistor in parallel of the capacitor, the circuit behaves like an inverting amplifier with a low frequency, and saturation is avoided.
The differentiator works similarly to the integrator by swapping the capacitor and the resistor.
Figure 1.9: Differentiator Amplifier
Figure 1.10: Schmitt trigger
In this configuration, the input voltage is applied through the resistor R1 (which may be the source internal resistance) to the non-inverting input and the inverting input is grounded or referenced. The hysteresis curve is non-inverting and the switching thresholds are 
where Vsat is the greatest output magnitude of the operational amplifier.
Figure 1.11: Instrumentation amplifier
Combines very high input impedance, high common-mode rejection, low DC offset, and other properties used in making very accurate, low-noise measurements.
It is made by adding a non-inverting buffer to each input of the differential amplifier to increase the input impedance.
Figure 1.12: Opamp as buffer
The buffer is an extremely useful circuit, since it helps to solve many impedance issues. The input impedance of the op-amp buffer is very high: close to infinity. And the output impedance is very low: just a few ohms.
This means we can use buffers to help chain together sub-circuits in stages without worrying about impedance problems. The buffer gives benefits similar to those of the emitter follower we looked at with transistors, but tends to work more ideally.