1. OSCILLATOR & Operational Amplifier
Unit- 3
OSCILLATOR & Operational Amplifier

2. Oscillators
An oscillator is an electronic circuit that converts direct voltage into alternating voltage of a desired frequency and magnitude without the use of mechanical moving parts.

3. TO PRODUCE OSCILLATION
An Oscillator uses transistor or vacuum tube in a circuit to generate an ac output.
It uses an oscillatory or tank circuit containing either L and C or R and C to produce oscillation.
A Feedback circuit to supply the energy to the tank circuit to sustain the output without the need for a signal from preceding stage.
The freq of the oscillator is determined by the condition that the loop phase shift is zero.

4. Introduction
Op-amps are basically a multi-stage, directly coupled high gain electronic voltage amplifier with a differential input and single ended output.
Negative feedback amplifier.
Uses Voltage Shut Feedback to provide a stabilized voltage gain.
It has High i/p impedance & Low o/p impedance.
It is used to amplify both AC & DC input signals.
Designed to perform mathematical operations ……..

5. Other applications Sign changing Scale changing Phase shifting
Voltage regulating
Analog computer operation
Instrumentations and control systems
Oscillator circuit
Pulse generator, square-wave generator, triangular wave generator, comparators, convertors etc………
It is manufactures with integrated Transistors, resistors, capacitors, diodes.

6. Advantages
Low cost
Small size
High reliability
Temperature stability
Low value of offset voltage and current
External components can be connected and characteristics can be changed.
Voltage gain of IC Op-amps are larger bcoz large voltage gain , externally connected resistors must be employed to provide negative feedback for most of the application.

7. Symbol

9. The input stage is differential amplifier.
Special techniques are used to provide the high input impedance necessary for the operational amplifier.
2nd stage high-gain voltage amplifier. To provide high gain.
Final stage is output amplifier stage. provides low output impedance.
The actual circuit used is emitter follower.
The output stage should allow the amplifier to deliver several milli-amps to a load.
The op-amp has +ve and –ve supply.
This makes the p-amp to produce positive or negative output.
[-] inverting input & [+] Non-inverting input.

10. Op-amps can be used in 3 different mode operations.
1st mode, with differential input both input terminals are used and two input signal which are 1800 out of phase with each other are used.
This produce an output signal that is in phase with signal on the non-inverting input.
2nd, the non-inverting input is grounded and a signal is applied to the inverting input. The output will be out of 1800 phase with the input signal.
3rd, the inverting input is grounded and the signal is applied to the non-inverting input. The output will be in phase with the input signal.

11. Op Amp Equivalent Circuit
vd = v2 – v1
A is the open-loop voltage gain
Voltage controlled voltage source

12. Equivalent circuit
used to analysis the basic operating principle of Op-amp and is observing effects of feedback arrangement.
The voltage source Avd is an equivalent Thevenin voltage source, Rout is the Thevenin equ resistance looking back into the o/p terminal of the op-amp.
The o/p voltage of the circuit is given
Vout = Avd =A[v1 – v2]

13. Voltage Transfer Characteristic
The voltage transfer curve may be defined as a curve which is drawn between input and output voltage.
For an op-amp the i/p voltage is Vid and o/p is Vo
Vout= [V+ -V-]Gopen-loop
The graphical representation is called the transfer curve of an op-amp.
The o/p voltage is dependent on the power
Supply voltage.
3 mode operation: V0=VCC [positive saturation]
[Linear region],-VCC < V0= Gvid < VCC
V0= -Vcc [negative saturation]

14. Op-amp
Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification
Used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation.
An operational amplifier is a direct coupled high gain amplifier consisting of one or more differential (OPAMP) amplifiers and followed by a level translator and an output stage. An operational amplifier is available as a single integrated circuit package.

15. Input stage is a differential amplifier and provides most of the voltage gain and also establishes the input resistance.
Intermediate stage is another differential amplifier which is driven by the output of the first stage.
Because direct coupling is used, the dc voltage level at the output of intermediate stage is well above ground potential. Therefore level shifting circuit is used to shift the dc level at the output downward to zero with respect to ground.
Output stage is a push pull complementary amplifier. It increases the output voltage swing and raises the current supplying capability. And provides low output resistance.
Commonly available and used of all operational amplifiers is μA-741.

16. Operational Amplifier is a three -terminal device which has two inputs, one called the Inverting Input, ( - ) sign and the other one called the Non-inverting Input, ( + )sign. And one output terminal.

17. Ideal op-Amp equivalent ckt
Zi [input impedance] =
infinity
Z0 [ output impedance]=
ZERO
A[ open loop gain] =
Input offset voltage [ Vio ]=
zero
Input offset Current [ Iio ]=
Bandwidth = infinity
Common mode rejection ratio = infinity

18. Ideal op-amp Characteristics
Infinite open loop gain.[when doing theoretical analysis, limit should be taken as open-loop gain goes to infinity]
Infinite bandwidth [the frequency magnitude response is flat everywhere with zero phase shift] The ideal op-amp will amplify all signals from DC to the highest AC frequencies.
Infinite input impedance.[impedance between its two inputs]
Zero input current.[there is no leakage or bias current in to the device]

19. 5. Zero input offset voltage [ when the input terminals are shorted V+=V- the output is virtual ground] 6. Infinite Slew rate [ the rate of change of the output voltage is unbounded]. The limiting rate of change of output voltage is called slew rate. 7. Zero output impedance.[Rout=0 the output voltage does not vary with output current] 8. Zero Noise 9. Infinite CMRR. [CMRR- the ability of a differential amplifier to reject interfering signals common to both inputs, and to amplify only the difference between the input] 10. Infinite power supply rejection ration for both power supply rails.

20. Op-Amps Configuration
In op-amp configuration, there is no connection between input and output and the op-amp works as a very high gain amplifier.
3 configurations
Non-Inverting amplifier
Inverting Amplifier
Differential Amplifier

21. Differential Amplifier
The differential amplifier amplifies the difference between two input voltage signals. It is called Differential Amplifier.
V0 = V1 – V2
In an ideal differential amplifier, the output V0 is proportional to the difference b/w the two i/p signals
V0 α V1 – V2

22. Differential Gain V0=Ad [ V1- V2]
Ad is the constant proportionality. The Ad is the gain with diff. amplifier amplifies the difference b/w two i/p signal. Its called differential gain of the differential amplifier.
V0= Ad.Vd
Hence the diff.gain is expressed as Ad=V0/Vd
The diff.gain is expressed in dB values as,
Ad=20 log10 [Ad] in dB

23. Inverting Amplifier i1 i virtual ground
In the inverting amplifier, the i/p is applied at the inverting terminal of the amplifier.
A feed-back connection b/w the o/p and the inverting input terminal produces a closed-loop circuit.
Inverting i/p terminal having different signal on it than the actual input voltage as it will be the sum of the i/p voltage plus the negative feed back voltages or summing point. i1 i .
virtual ground

24. G=Vout / Vd
Vd=V1 –V2
G= Vout/V1 –V2 [open loop gain is infinite for op-amp]
V1 –V2 =V0 / ∞=0
V1 –V2 =0 [ the potential difference b/w two terminals is zero] virtually short circuit.(the voltage at non-inverting terminal, it will automatically appear at the inverting terminal because of the infinite voltage gain G)
V1 = V2

26. Non-inverting Amplifier

27. The formula to calculate the output voltage of a potential divider network, we can calculate the output voltage gain of the Non-inverting amplifier.
V2=[R1 / R1 +Rf]
Voltage gain[G]: Vout/ Vin
V2 = Vin
The non-inverting amp
Is never less than unity.

28. Common Mode Rejection Ratio [CMRR]
Most forms of interference, static, induced voltages, noise etc.. Drive a differential amplifier in the common mode.
These unwanted signals should not be present at the output. The differential amplifier has the property of rejecting such common mode signals.
Therefore, CMRR is the ability of a differential amplifier to reject a common-mode signals such as noise.
“the ratio of differential mode gain to the common mode gain. CMRR=

29. Slew Rate
“ the maximum rate of change of output voltage per unit of time and is expressed in volts per microsecond”
Slew rate, SR=
The slew rate is dependent upon the high-freq response of the amplifier stages with in the op-amp.
It indicates how rapidly the o/p of an op-amp can change in response to change in the input frequency.
SR= Imax / C “voltage at which the capacitor rise is dVc/dt= I/C.