INTRODUCTION:
- The operational amplifier is the amplifier which can be used to perform various mathematical operations and find its use in analog computing.
- Basically, it is a multistage amplifier circuit which uses a number of amplifier stages interconnected to each other in a complicated manner.
- It has an exceptionally high voltage gain of around 10^5 or 10^6 dB.
- Circuit symbol of ICuA741 OpAmp.:
- We can apply input voltage which is to be amplified to any one of the input terminals while connecting another pin to ground or we can connect the input signal between the two input pins differentially.
- Vcc to OpAmp is connected to positive terminal of one source and –Vee is connected to the negative terminal of another source.
- An ideal OpAmp is expected to amplify the differential signal present between its two input terminals.
- The differential amplifier is used in many applications where the response from DC (Zero hertz frequency) to megahertz is required.
BLOCK DIAGRAM OF A TYPICAL OPAMP:-
- Input Stage: Consist of dual input balanced output. This stage provides the maximum gain to OpAmp and also determine the input impedance value Ri.
- Intermediate stage:- Consist of a dual input unbalanced output. This stage is driven by input stage single ended output.
- Level Shifting Stage:- Due to the direct coupling used between first two stage. The input to this stage is an amplified waveform at some non-zero DC level. Hence, this stage brings to offset back to zero volts with respect to ground.
- Output:- Increases the magnitude of the voltage and current capacity of the OpAmp while lowering the output resistance.
OPAMP INPUT MODES:-
Single Ended Mode: If the input voltage is given at only one of the inputs of the OpAmp , while the other input is grounded then we call such configuration of OpAmp as Single Ended Mode.
Inverting
Non-Inverting
Differential Mode: If the OpAmp is given two voltages with a phase shift (180 degrees) then the difference between the two input is been amplified. Also known as Dual (Double) Ended Mode.
Common Mode: If both the inputs are connected to single input voltage then ideally the output should be zero.
CMRR: Common Mode Rejection Ratio is the ability of the OpAmp to reject the common mode signal. Also known as the figure of merit.
CMRR is the ratio of open loop gain to the common mode gain.
Ideally, its value should be infinity; practically it should be as high as possible.
CMRR = (Av/ Acm) ;
CMRR = 20 log(Av/Acm) dB;
OPAMP PARAMETERS:
Open Loop Gain:
open loop mode of OpAmp is one in which no feedback is provided to the OpAmp.
The gain of the OpAmp in such a mode is called Open Loop Gain.
Mathematically, it is given by
Vo = Av * Vd.
Differential Input Resistance (Ri):
It is also called as input resistance.
It is defined as the equivalent resistance measured at either of the OpAmps input terminals provided other is grounded.
Ri should be as high as possible and ideally, it is considered to be infinity.
For u741A OpAmp it is in few MegaOhms.
For OpAmps with FETs, it goes as high as few thousand GigaOhms.
Output Resistance (Ro):
It is defined as the equivalent resistance as seen from the output terminal of the OpAmp while the input terminals are short-circuited.
Ro is measured with respect to ground.
Ro should be as small as possible and ideally it should be zero. Because this ensures that output voltage regulation works properly and improves current carrying capacity.
Input Capacitance (Ci):
It is defined as the equivalent capacitance measured at either of the input terminals of the OpAmp, provided another input is grounded
Ci should be as low as possible.
Input Offset Voltage (Vios):
Ideally, for zero input voltage, there should be a zero output voltage. But practically it isn’t so.
This is due to the unavoidable imbalance for at differential input block of the OpAmp.
Hence, the voltage required at the input to make the output zero is called input offset voltage.
It is temperature dependent and Vios should be as low as possible. Ideally, it should be zero.
Output Offset Voltage (Voos):
It is defined as the voltage produced at the output due to the input offset voltage.
A compensation network is used to nullify the effect.
Input Bias Current (Ib):
It is defined as the average of the input currents flowing through the input terminals of the OpAmp.
Ib = ( Ib1 + Ib2 ) / 2.
Ideally, it must be zero and practically it must be as low as possible.
This current does exist due to the finite value of input resistance Ri.
And as the two transistors in the OpAmp aren’t same the currents flowing through the two input terminals are also different.
This value can be reduced to PicoAmperes if we use FET operated OpAmp.
It is a temperature dependent quantity.
Input Offset Current (Iios):
It is defined as the algebraic difference between the currents flowing into its inverting and noninverting terminals.
| Iios | = | Ib1 – Ib2 |.
Ideally, it should be zero; practically it should be as low as possible.
Iios exist due to the unequal currents Ib1 and Ib2 flowing through the terminals of the OpAmp.
FET operated OpAmp give lower values of this current as compared to BJT.
This current is responsible for giving finite output offset voltage.
It is a temperature dependent quantity.
Common Mode Rejection Ratio (CMRR) :
It is defined as the ratio of differential gain to common mode gain.
CMRR = (Av/ Acm) ;
CMRR = 20 log(Av/Acm) dB;
It indicates the capability of OpAmp to reject the common mode signals. Example : Noise.
Generally, it is expressed in dB (decibels).
Ideally, this ratio should be infinite, and practically it needs to be as high as possible.
Power Supply Rejection Ratio (PSRR):
It is defined as the change in input offset voltage to the variation in the supply voltage.
PSRR = (delta) Vios / (delta) V.
Also called as Supply Rejection Ratio (SVRR).
Expressed in microvolts per volts or decibels
Ideally, its value should be zero; practically it should be as low as possible.
Slew Rate (S):
It is defined as the ratio of the maximum rate of change of output voltage to unit time.
It is expressed in volts per microseconds.
S = (delta)Vo / (delta)t. [Vo = max].
Importance of Slew Rate :
Signifies the capability of the OpAmp to change its output rapidly. Hence, decides the highest frequency of operation without giving any distortions at the output.
Fm = S / 2 * pi * Vm.
Fm = S / 2 * pi * Vm.
It depends on the change in voltage gain. Hence, it is always specified at unity gain.
Ideally, it should be infinite; practically it should be as high as possible.
Bandwidth:
It is defined as the range of frequencies that can be amplified almost equally.
It should be as large as possible
Frequency response should extend right up to zero hertz.
Ideally gain of the OpAmp should be constant from zero to infinite hertz.
Gain Bandwidth Product:
It is defined as the bandwidth of the OpAmp at unity gain.
Also called as
Closed Loop Bandwidth.
Unity Gain Bandwidth.
Small Signal Bandwidth.
Large Signal Voltage Gain:
It is defined as the ratio of output voltage to input differential voltage.
Av = Vo / Vd ; Vd = V1 – V2.
Output Voltage Swing:
It is defined as the maximum positive or negative output voltage of an OpAmp is called saturation voltage.
+ - Vsat = Vo(max).
typically Vsat = 0.9 * Vcc.
Output Short Current:
OpAmp’s output should never be short-circuited to the ground because if we do so large uncontrolled current flow through OpAmp which could damage it.
To avoid this every OpAmp has an inbuilt short circuit protection circuit which would limit the current to a safer value. Typically it is 25mA.
Parameters
|
Dependency
|
Voltage Gain
|
Supply Voltage, Frequency
|
Power Consumption
|
Supply Voltage, Temperature
|
Input Offset Current
|
Supply Voltage, Temperature
|
Common Mode Input Voltage Range
|
Supply Voltage
|
Output Voltage Swing
|
Supply Voltage
|
Input Resistance
|
Frequency, Temperature
|
Output Resistance
|
Frequency
|
CMRR
|
Frequency
|
Input Bias Current
|
Temperature
|
Absolute Power Dissipation
|
Temperature
|
Slew Rate
|
Temperature
|
Gain Bandwidth Product
|
Temperature
|
CHARACTERISTICS OF AN IDEAL OPAMP:
Ri = infinity.
Ro = 0.
Av = infinity.
Bandwidth = infinity.
CMRR = infinity.
Slew Rate = infinity.
Vios = 0.
PSRR = 0.
~Jay Mehta
