The operational amplifier is a high gain integrated directly coupled amplifier that can perform a large number of linear and non-linear amplifications and signal processing functions. Generally an amplifier has the following stages.
a. Input stage
b. Intermediate stage
c. Level shifting stage
d. Output stage
The first stage of an op-amp is almost a differential amplifier and the lase stage is usually a Class B Push-pull emitter follower.
The input stage is a dual input and balanced output differential amplifier. This stage provides most of the voltage gain of the amplifier and also establish the input resistance of the op-amp. The input stage of the Differential amplifier determines the input characteristics of an operational amplifier. The basic requirements of input stage is listed below
- High input impedance (±10 k Ω)
- Low input bias current (<0.5 μA)
- Less input off set voltage (<10 mV)
- Less input offset current (<0.2 mA)
- High CMRR (≥70 dB)
In most of the amplifiers, an intermediate stage (dual input, unbalanced output differential amplifier) is provided which increases the overall gain of the op-amp. Because of direct coupling between the first two stages, the de level at the output of the intermediate stage is well above the ground potential. This requires a level translator at the succeeding stage in order to bring the de level back to the ground potential.
Level shifter stage
The level shifter circuit is used after the intermediate stage to shift the de level at the output of intermediate stage downward to zero volts with respect to ground. Usually the third stage is an emitter follower using a constant current source.
The last stage is a complementary symmetry push-pull amplifier. The requirement of output stage is listed below,
- High output current and voltage using capability
- Low output impedance
- Short circuit protection.
- Less stand by power
OP-AMP SYMBOL AND TERMINALS
The symbol for an op-amp along with its various terminals
The op-amp is indicated basically by a triangle which points in the direction of the signal flow.
All the op-amps have at least following five terminals.
1. The positive supply voltage terminal Vcc or V
2. The negative supply voltage terminal – VEE or -V
3. The output terminal
4. The inverting input terminal
5. The non-inverting input terminal
The input at the inverting terminal is positive voltage, the output voltage is negative voltage and vice versa, while the input at non-inverting terminals results in the same polarity output. The input and output are in antiphase means having a 180° phase difference in between them while the in phase input and output means is having a 0° phase difference in between them.
The op-amp works on a dual power supply. A dual power supply consists of two supply voltages both dc.
The dual power supply is generally balanced (i.e.) the voltage of the positive supply +Vcc and that of the negative supply-VEE are same in magnitude. The typical commercially used power supply voltages are 15 V. But if the two voltages magnitudes are not equal in a dual supply, it is called unbalanced dual power supply. Practically in most of the op-amp circuits balanced dual supply is used. The other popular balanced dual supply voltages are ±9 V, ±12 V, ±22 V etc. The balanced and unbalanced dual supplies
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