With respect to op-amp explain the ideal characteristics and concept of virtual aground. Explain how op-amp can be used as an averaging amplifier in inverting configuration. Also draw neat circuit diagrams to - I. convert sine wave to square wave in op-amp. - II. Detect the crossing of zero’s in the generated square wave. -

Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: Dec 2015

Ideal Characteristics of Opamp:

1. Infinite input impedance
2. Zero output impedance
3. Zero common-mode gain, or, infinite common-mode rejection
4. Infinite open-loop gain A
5. Infinite bandwidth.

Concept of Virtual Ground

1. An Op-Amp has a very high gain typically order of $10^5$ .
2. If power supply voltage Vcc =15V. Then maximum input voltage which can be applied $$V_d = Vcc /A_d = 15/ 10^5 = 150 µ V$$ i.e. Op -Amp can work as a linear amplifier (from $+V_i to –V_i$) if input voltage is less than 150 µV. Above that Op-Amp saturates.
3. If $V_1$ is grounded then $V_2$ can not be more than 150 µV which is very small and close to ground.
4. Therefore $V_2$ can also be considered at ground if $V_1$ is at ground. Physically $V_2$ is not connected to the ground yet we considered $V_2$ at ground that is called virtual ground.

Sine wave to Square wave

1. A comparator is a two-state output circuit, the output being either a low or a high.
2. The input voltage at which the output changes state either from low to high or high to low is called the trip-point, threshold or reference.
3. For the opamp comparator the trip-point is zero and hence they are called zero-crossing circuits. So it is obvious that a periodic voltage waveform at the input of a zero crossing detector will result in a rectangular waveform at the output.
4. For example, if a sine wave is applied to the input of a non-inverting comparator with 0 V reference, it will result in a square wave output as shown in Fig. (a).
5. The output switches states whenever the input voltage crosses the reference voltage of 0 V. Fig. (b) shows sine wave to square wave conversion using an inverting zero-crossing detector. Note that the output square wave is 180° out of phase with the input sine wave.