A Study on Op-amp
By Ayushi Agrawal , Central University Bhatinda
The aim of this Term Paper is to provide basic and essential knowledge of Op-amp.
In this paper I shall try to compile analog electronics based on Operational Amplifiers and also the knowledge about how these circuits are essential.
The use of Operational amplifiers are very wide and particularly used in every communication system where we need to amplify a signal w.r.t to its voltage or power.
After studying this paper we will be able to understand :
1.What is an operational amplifier?
2.History of operational amplifiers.
3.How an operational amplifier works?
4.Advantages of Op-amps over other amplifiers.
5.Uses of Op-amps in analog and digital electronics.
Operational Amplifier is a monolithic linear integrated circuit which is DC coupled and has a very high differential gain. It is a voltage control device that means by varying a small voltage values we can have our desired output voltage or currents.
History Of Op-Amps :
In 1941 when transistors were not introduced, op-amp was made of Vacuum tubes. It was just a summing amplifier that had 90dB gain. And it required a lot of +350 and -350 volts to operate. It remained in fashion till 1963.
In 1963, the first monolithic IC uA 702 designed by Fairchild Industries made op-amps practical and replaced the vacuum tubes.
Later in late 1968 uA741 ICs were introduced that had inbuilt transistors assembly of transitors acting as op-amps having two input terminals and one output terminal.
And after one year of release, FET and MOSFET were built and hence they improved the efficiency op-amps in speed and power consumption.
Basics of Op-amp :
?Infinite open-loop gain G = vout / vin
That is , an op-amp when used in open loop gain then it has a very large value. But in practical op amps it has a finite value.
?Infinite input impedance Rin, and so zero input current
In op-amp there is infinite resistance between the two input terminals called input impedance. However in practical op-amp this value is finite and ranges from 10^5 to 10^6
?Zero input offset voltage
When op-amp is used we must have zero output if we do not apply any input voltage at all. So in ideal op-amp there is almost zero output voltage when no input signal is applied and in practical op amps there is few milli volts offset voltage is noticed.
?Infinite output voltage range
When op amp is used as amplifier , due to its high gain the output voltage is very high so it has large output voltage range.
?Infinite bandwidth with zero phase shift and infinite slew rate
The ability to operate over a large range of frequency is called bandwidth. An op-amp can operate from zero frequency that is a DC current to AC current with frequency 10^5. And can maintain zero phase difference for purpose. The rate of change of output voltage as soon as the input voltage is changed is called slew rate and this is essential for very fast operation.
?Zero output impedance Rout
An ideal op-amp literally offers no resistance at all at output terminals. However practical op-amps do offer few ohms of resistance at the output terminals.
Op-amps have transistor built internally and there is no humming effect during the operation. Thus op-amp does not create any noise at all.
?Infinite common-mode rejection ratio (CMRR)
When one of the input terminal is grounded and both terminals are connected to each other so there is no voltage difference between two terminals op-amp note is as zero difference in voltage and gives zero output.
These ideals can be summarized by the two “golden rules”:
I.In a closed loop the output attempts to do whatever is necessary to make the voltage difference between the inputs zero.
II.The inputs draw no current.
The first rule only applies in the usual case where the op-amp is used in a closed-loop design (negative feedback, where there is a signal path of some sort feeding back from the output to the inverting input). These rules are commonly used as a good first approximation for analyzing or designing op-amp circuits.
OP AMP symbol , IC and INETRANL CIRCUITARY
An equivalent circuit of an operational amplifier that models some resistive non-ideal parameters.
?Pin-1 is Offset null.
?Pin-2 is Inverting (-) i/p terminal.
?Pin-3 is a non-inverting (+) i/p terminal.
?Pin-4 is -Ve voltage supply (VCC)
?Pin-5 is offset null.
?Pin-6 is the o/p voltage.
?Pin-7 is +ve voltage supply (+VCC)
?Pin-8 is not connected.
Stage : 1
Differential Amplifier : It amplifies the weak input signal. Here input signal is fed into the circuit and common emitter high gain configured transistors amplify the given signal up to 1000 times.
Now the term differential amplifier means that it will amplify the difference of two input signal given. Blue assembly in the diagram shows the differential input stage.
Stage 2 :
Intermediate stage : It is also known as second amplifying stage or additional gain stage. As the name suggests this stage is used to have more Gain. The necessary Gain can not be achieved by only using one stage amplifier so another assembly of transistors pink colored is intermediate stage. It provides additional 100 times Gain.
Both stages are connected by emitter follower transistors for good impedance matching and direct coupled.
Stage 3 :
Green colored transistor is constant current sourced voltage level shifting stage. After certain level of amplification the voltage is fed to the next stage with direct coupling method but due to transistors operation there appears a small DC voltage in amplified signal. All transistors are working in active region so small voltage can turn transistors into cut-off region and thus operation will be broken. Thus a Clamper circuit is used, the Cc capacitor called compensating Capacitor.
This voltage shifting stage removes the DC component from the signal and provides clear AC signal for the output purpose.
Stage 4 :
This is the last stage of amplification called Power Amplifier.
We can not directly fed our amplified signal into any load because the output of the voltage shifter has a very high resistance. This resistance cause trouble in power transfer.
Thus the last stage of the amplifier is used to transfer maximum power to the load.
NOTE this stage does not amplify power or signal, it just transfer maximum power to the load via impedance matching mechanism.
Internal transistors operation :
First step to understand the OP-AMP internal operation is to understand the biasing circuit of the whole assembly. In this assembly a branch called biasing branch is used to operate all transistors and give them constant currents using CURRENT MIRRORS. The red colored transistors are current mirrors. Current mirrors are the circuit that mimics the current from another branch of the circuit.
Voltage drop across resistor is equal to
This voltage is now fed up to Q10 and thus Q3, Q4.
Next stage is differential input stage. Current from Q10 and Q11 is delivered to the base of Q3 and Q4 transistors and form the differential input stage.
Q1 and Q2 are connected as emitter follower and protect Q3 and Q4 from breakdown. Together Q3 and Q4 offers high input impedance and thus high amplification level.
Q5, Q6, Q7 and three resistors creates output of the first stage. High load converts differential signal to single ended signal and provides high gain. Then single ended output is taken out from collector of Q6.
This signal is then fed into Q15 base. Q15 is emitter follower that gives 2nd stage amplification. And Q16 is again common emitter follower which minimizes the loading effect and prevents gain loss. Next output of Q16 is used to fed into the last stage of amplifier.
One concept is mostly used here is ACTIVE LOAD CONCEPT, the use of transistor current source as a load resistance. This gives high gain without high load resistance and saves chip area.
Since there will be no high load resistance there is no need for high supply voltage. E.g; is the load on Q16 collector.
A capacitor is used in the circuit is called Miller compensation capacitor is connected into feedback path creating a dominant pole of approx 5Hz and provides frequency compensation in the amplification and that shifts other poles of the amplifiers to reduce the DC levels.
Last is the output stage of the Op-amp.
Q14 is called source transistor and Q20 is called sink transistor. Together they form the output complementary stage. Both transistors are equal in area and fairly larger than other transistors because they transfer maximum current with minimal temperature effect on the device.
If output of the amplifier is positive then Q14 pulls the more power and makes the output more positive.
And if the output is negative then Q20 pushes the more power and hence making it more negative on the output stage. This is called PUSH-PULL stage of amplifier.
This amplifier has short circuit protection, not all amplifiers has short circuit protection. Q17 is current limiting protector , this protects the Q14 from short-circuit and Q19 for Q20.
OP-AMP USES :
1.In open loop configuration :
When there is no direct connection between input and output terminals of the op-amp then it is said to be in open loop configuration.
So op-amp is used as comparator in open loop configuration.
Comparator : A comparator is a circuit which compares a signal voltage applied at one input of an op-amp with output ±Vsat = (Vcc). If the signal is applied to the inverting terminal of the opamp it is called inverting comparator and if the signal is applied to non-inverting terminal of the op-amp it is called non-inverting comparator. In an inverting comparator if input signal is less than reference voltage, output will be +Vsat. When input signal voltage is greater than reference voltage output will be –Vsat. The vice-versa takes place in non-inverting comparator.
In theory, a standard op-amp operating in open-loop configuration (without negative feedback) may be used as a low-performance comparator. When the non-inverting input (V+) is at a higher voltage than the inverting input (V-), the high gain of the op-amp causes the output to saturate at the highest positive voltage it can output. When the non-inverting input (V+) drops below the inverting input (V-), the output saturates at the most negative voltage it can output. The op-amp’s output voltage is limited by the supply voltage. An op-amp operating in a linear mode with negative feedback, using a balanced, split-voltage power supply, (powered by ± VS) has its transfer function typically written as: . However, this equation may not be applicable to a comparator circuit which is non-linear and operates open-loop (no negative feedback)
2.In Close loop configuration :
When there is a connection between input and output terminals by means of active or passive components then it is called close loop configuration.
A)When output is connected to non-inverting terminal it is called Schmitt-Trigger circuit. It is used to generate square wave.
B)When Output is connected to inverting terminal
1. Inverting amplifier : when output is connected with input as shown in figure the output voltage is noticed with 180 degree phase out. Thus it is called inverting amplifier.
Here Rf = R2 and Rin = R1
2 . Non-Inverting Amplifier : in this circuit the input is fed into the non inverting terminal and output is connected to inverting terminal.
3.Summing amplifier : any number of voltage and resistors are connected in such a way so that output is algebraic sum of all individual voltages.
4.Differentiator : when the output signal is derivative of the input signal or an high pass filter is called differentiator. Circuit diagram shows differentiator.
5.Integrator : when the output signal is anti-derivative of the input signal or low pass filter is called integrator.
6.Voltage follower : A buffer circuit or voltage follower does not amplify or invert the signal but isolate the one part of circuit to the other.
7.Multivibrator : Instead of a sinusoidal waveform being used to trigger the op-amp, we can use the capacitors charging voltage, Vc to change the output state of the op-amp.Once the op-amps inverting terminal reaches the new negative reference voltage, -Vref at the non-inverting terminal, the op-amp once again changes state and the output is driven to the opposing supply rail voltage, +V(sat). The capacitor now see’s a positive voltage across its plates and the charging cycle begins again. Thus, the capacitor is constantly charging and discharging creating an astable op-amp multivibrator output.