Site icon Hackatronic

Inverting amplifier (OPAMPs)

Abhishek Singh

The inverting amplifier is one of the types of closed loop opamp configuration. In this configuration, the non-inverting (+ve) terminal of the opamp is connected to the ground, while the inverting terminal acts as input terminal. Negative feedback is present between the output terminal and (-ve) inverting terminal of OPAMP.

Importance of negative feedback in Inverting amplifier:

The open loop gain of an operational amplifier is very high up to 120 dB. Practically this gain is unstable and difficult to control.
Get SEO Backlinks

If we use the OPAMP in open loop configuration it will be highly sensitive to a small input in (μV) microvolts, and the output will swing between +Ve high and -Ve low.

In order to control the gain of the amplifier, we introduce negative feedback. As the gain of an amplifier is very high, we can sacrifice some gain for stability. Negative feedback is the most common way to stabilize amplifier gain. In this method by using a resistor Rf some part of the output is feedback to the negative input terminal.

This -Ve feedback produces stability to the output of the amplifier.

Concept of negative feedback:

Negative feedback is the process of feeding back output to the input terminal of the operational amplifier. To make feedback negative it is applied to -ve inverting terminal. Due to this, the differential voltage becomes smaller.

Whenever there is a change in input the output also changes due to this the negative feedback also changes which stabilizes the gain, making a closed loop. That’s why the gain is called closed-loop gain.

The voltage appearing at the inverting terminal is the sum of the input signal and -Ve feedback signal, Making a summing point. Hence, we must separate the input signal from the feedback signal by using a resistor between input and summing point.

The inverting amplifier:

Inverting amplifier circuit

Operation of OPAMP circuit:

Invertin amplifier waveform

Expression for the closed-loop voltage gain:

From the circuit, we can say that

Vo = [Av]×Vd  
Vd = [Av] ÷ Vo

Where Av = open-loop gain

As we know that open loop gain of OPAMP is ∞,

Vd = Vo ÷ 0 = 0
Vd = V1 - V2
since, Vd = 0
V2 - V2 = 0

As the +ve input terminal is connected to ground, V1 = 0

thus, V2 = 0

Vs = IR1
Vo = -IRf

V2 is approximately at ground potential.

Closed loop gain

Avg = Vo /Vs

Substituting the values of Vs and Vo we get,

Avf = IRf /IR1 = - Rf/R1
Vo = Avf *Vs

The conclusion from the above expression:

  1. The value of closed-loop voltage gain Avf does not depend on the value of open-loop voltage gain Av.
  2. the value of closed-loop gain can be adjusted by wearing the value of resistors Rf and R1. Rf is a potentiometer to adjust the gain.
  3. The output voltage is amplified and inverted version of the input voltage.

7490 Decade Counter Circuit (Mod-10) Designing

 

Exit mobile version