ELEC207 LINEAR INTEGRATED CIRCUITS

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1 Concept of VIRTUAL SHORT For feedback amplifiers constructed with op-amps, the two op-amp terminals will always be approximately equal (V + = V - ) This condition in op-amp feedback amplifiers is known as the virtual short. This is to avoid saturation as gain of the amplifier is ideally infinitely large. It appears that the two input terminals are shorted together ( Not shorted in real. It is the feedback that enforces short). If a true short were present, then current could flow from one terminal to the other. However, we know that the input resistance of an op-amp is ideally Infinite and thus we know that the input current into an op-amp is zero. Conclusion from Virtual short Concept The voltage difference between Non-inverting and Inverting terminal is zero V + = V - The current into both Non-inverting and Inverting terminal is zero. I + = 0 and I - = 0

2 Concept of VIRTUAL SHORT V 2 A = V 1 Applying the concept of a virtual short we can simplify the analysis of an op-amp feedback amplifiers

3 Inverting Amplifier Input is applied to the inverting terminal of Op-amp Non-inverting terminal is grounded Output voltage is out of phase with the input voltage by 180 degree or is of opposite polarity V 0 = - A V in Non-Inverting Amplifier Input is applied to the non-inverting terminal of Op-amp Inverting terminal is grounded Output voltage is in phase with the input voltage or is of same polarity V 0 = A V in A practical op-amp alone cannot be used as an amplifier with controlled gain and is limited to comparator applications. External resistors are therefore connected to the op-amp in a feedback arrangement to set the gain of the amplifier.

4 Analysis of INVERTING AMPLIFIER using the concept of VIRTUAL SHORT We know, We know, If R F = R in,then V out = -V in

5 Analysis of NON-INVERTING AMPLIFIER using the concept of VIRTUAL SHORT Using Potential divider Rule:

6 EXAMPLES: 1. Find the closed loop gain of the following circuit. 2. If R in is 10kΩ, what value of R f is required to produce a non-inverting amplifier with voltage gain of 25?

7 SOLUTION: 1. Find the closed loop gain of the following circuit. 2. If R in is 10kΩ, what value of R f is required to produce a non-inverting amplifier with voltage gain of 25?

8 11/02/2014 VOLTAGE FOLLOWER (NON-INVERTING BUFFER/UNITY GAIN BUFFER) When a Non-Inverting amplifier is configured for unity gain, it is known as a Voltage follower In a Voltage follower output follows the input To obtain the voltage follower from the Non-Inverting amplifier, simply open R 2 and short R F As the input signal is connected to the non-inverting input of the amplifier the output signal is in phase with the input. V o = A V in V o = V in Since A=1

9 11/02/2014 What is the Purpose of a Voltage Follower? According to ohm's law, current, I=V/R. If a load has very low resistance, it draws huge amounts of current. This causes huge amounts of power to be drawn by the power source and because of this, there will be high disturbances or noise. Voltage follower draws very little current from the power source because of the high input impedance of op-amp. A voltage follower does not disturb the original circuit, and give the same voltage signal as output. They act as isolation buffers, isolating a circuit so that the power of the circuit is disturbed very little.

10 11/02/2014 INTEGRATOR Replace feedback resistor R F by a capacitor C F in a basic inverting amplifier The output voltage V o is inversely proportional to the negative of R 1 C and proportional to the integral of the input voltage V IN with respect to time. The minus sign (-) indicates a phase shift between input and output. Integrator Circuit Integrator input and output waveforms

11 11/02/2014 INTEGRATOR

12 11/02/2014 PRACTICAL INTEGRATOR When V in =0, Capacitor acts as a open circuit Ideal integrator act as a open loop amplifier Output will be a error voltage because of the input offset voltage To reduce the error voltage at the output, a resistor R f is connected across the feedback capacitor C f in a practical integrator.

13 11/02/2014 INTEGRATOR FREQUENCY RESPONSE AND DESIGN CONSIDERATIONS f b is the frequency at which gain is 0 db. In the response, till f a, gain is constant and after f a, gain is decreasing. In the region between f a and f b it acts as a integrator. Choose f b equal to frequency of the signal to be integrated. R f C f and R 1 C f values must be selected such that f a < f b. Ex: If f a = f b /10, then R f =10 R 1 The input signal will be integrated properly if Time period of the signal