UNIT 5 More Applications

1. Subject Name: LINEAR IC’s AND APPLICATIONSSubject Code:10EC46Prepared By: Kumutha ADepartment: Electronics and CommunicationDate:30-3-2015

2. UNIT 5More Applications

3. Clamping Circuits • Peak detectors • Sample and hold circuits • Log and antilog Amplifiers • Multiplier and divider • Triangular and rectangular wave generators • Phase shift Oscillator • Wein bridge Oscillator

4. Clampers circuits In clamper circuits, a predetermined dc level is added to the input voltage. In other words, the output is clamped to a desired dc level. If the clamped dc level is positive, the clamper is called a positive clamper. On the other hand, if the clamped dc level is negative, it is called a negative clamper. The other equivalent terms for clamper are dc inserter or dc restorer. A clamper circuit with a variable dc level is shown in fig. Here the input wave form is clamped at +Vref and hence the circuit is called a positive clamper. The output voltage of the clamper is a net result of ac and dc input voltages applied to the inverting and non-inverting input terminals respectively. Therefore, to understand the circuit operation, each input must be considered separately. First, consider Vref at the non-inverting input.

5. The output voltage of the clamper is a net result of ac and dc input voltages applied to the inverting and non-inverting input terminals respectively. Therefore, to understand the circuit operation, each input must be considered separately. First, consider Vref at the non-inverting input. Since this voltage is positive, is +VO is positive, which forward biases diode D1. This closes the feedback loop and the op-amp operates as a voltage follower. This is possible because C1 is an open circuit for dc voltage. Therefore VO = Vref. As for as voltage Vin at the inverting input is concerned during its negative half-cycle D1 conducts, charging C1 to the negative peak value of the VP. However, during the positive half-cycle of Vin diode D1 is reverse biased and hence the voltage VP across the capacitor acquired during the negative half-cycle is retained. Since this voltage VP is in series with the positive peak voltage VP, the output peak voltage VO=2VP. Thus the net output is Vref+VP, so the negative peak of 2VP is at Vref. For precision clamping C1Rd<<T/2, where Rd is the forward resistance of the diode D1 (100Ω typically) and T is the time period of Vin. The input and output wave forms are shown in fig.

6. Peak Detector Circuit Square, triangular, saw-tooth and pulse waves are typical examples of non-sinusoidal wave forms. A conventional ac voltmeter cannot be used to measure the rms value of the pure sine wave. One possible solution for this problem is to measure the peak values of the non-sinusoidal wave forms. Fig shows a peak detector that measures the positive peak values of the input.

7. Sample and hold circuit

8. The voltage V’ would ideally stay constant in the hold mode by having a value equal to Vin at the instance of clock going low. But there are two error sources due to switch: 1.The channel charge go to both junctions to causes negative glitches. If source impedance of Vin is small, then the glitch is small and last a short duration. 2.The channel charge go to V’ causes a negative voltage that is long lasting.

9. An improved sample and hold circuit by using two OpAmps. Here the hold capacitor is placed in the feedback path of a second OpAmp. The advantage is that now both sides of Q1 is almost signal independent if OpAmp2 has a large gain. So, when Q1 turns off, there is still charge going to capacitor, but the Vout will have only a fixed DC error voltage.

10. Log Amplifier • Log Amplifier • Logarithmic amplifier gives the output proportional to the logarithm of input signal. • The fig shows log amplifier. • Derivation gives output voltage of log amplifier.

11. Log Amplifier

12. Log Amplifier • The previous circuit has some problem. The emitter saturation current Is varies from transistor to transistor and with temperature . • Thus stable reference voltage cannot be obtained. This Is eliminated by modification in circuit as shown. The two transistor are • Integrated close together in same silicon wafer, This provide a close match of saturation currents and ensure good thermal tracking.

13. Antilog Amplifier • The output is proportional to the antilog of input voltage.

14. Antilog Amplifier

15. Multiplier • The output voltage can be written as • If both inputs are positive, then the multiplier is called a one-quadrant multiplier. • If one input is kept at a positive value and the other input is allowed to take either a positive or negative value, then it is called a two-quadrant multiplier. • If both the inputs are allowed to take either positive or negative values, then it is called a four-quadrant multiplier. • There are some commercially available multiplier ICs and multiplier circuits can be constructed from op-amp ICs such as 741. • The applications of multipliers include frequency doubling, frequency shifting, phase angle detection, real power computation, and squaring signals.

16. Divider • The divider circuit is shown in fig. • The output can be written as • The applications of dividers include taking square root and dividing one number by another.

17. Triangular/ Rectangular wave generator • The circuit consist of a triangular and a inverting Schmitt trigger circuit. • The Schmitt output is applied as a input to the integrator and integrator output is applied as Schmitt trigger input. • When the Schmitt trigger output is positive the integrator output goes negative until it reaches the Schmitt LTP. • The Schmitt Output switches to negative and integrator Output then moves in a positive direction to the UTP of Schmitt trigger.(refer the waveform).

18. RC phase Shift Oscillator RC phase shift oscillator is a sinusoidal oscillator used to produce sustained well shaped sine wave oscillations. It is used for different applications such as local oscillator for synchronous receivers, musical instruments, study purposes etc. The main part of an RC phase shift oscillator is an opamp inverting amplifier with its output fed back into its input using a regenerative feedback RC filter network, hence the name RC phase shift oscillator.

19. RC phase Shift Oscillator • The feedback network offers 180 degrees phase shift at the oscillation frequency and the op amp is configured as an Inverting amplifier, it also provide 180 degrees phase shift. Hence to total phase shift around the loop is 360 degrees, it is essential for sustained oscillations. • At the oscillation frequency each of the resistor capacitor filter produces a phase shift of 60° so the whole filter circuit produces a phase shift of 180°. • The energy storage capacity of capacitor in this circuit produces a noise voltage which is similar to a small sine wave, it is then amplified using op amp inverting amplifier. • By taking feedback, the output sine wave also attenuates 1/29 times while passing through the RC network, so the gain of inverting amplifier should be 29 in order to keep loop gain as unity. • The unity loop gain and 360 degree phase shift are essential for the sustained oscillation. • RC Oscillators are stable and provide a well shaped sine wave output with the frequency being proportional to 1/RC and therefore, a wider frequency range is possible when using a variable capacitor. • However, RC Oscillators are restricted to frequency applications because at high frequency the reactance offered by the capacitor is very low so it acts as a short circuit.

20. Wein bridge oscilltor • The output of the operational amplifier is fed back to both the inputs of the amplifier. One part of the feedback signal is connected to the inverting input terminal (negative feedback) via the resistor divider network of R1 and R2 which allows the amplifiers voltage gain to be adjusted within narrow limits. The other part is fed back to the non-inverting input terminal (positive feedback) via the RC Wien Bridge network. • The RC network is connected in the positive feedback path of the amplifier and has zero phase shift a just one frequency. Then at the selected resonant frequency, ( ƒr ) the voltages applied to the inverting and non-inverting inputs will be equal and “in-phase” so the positive feedback will cancel out the negative feedback signal causing the circuit to oscillate.

21. Wein Bridge Oscillator • The voltage gain of the amplifier circuit MUST be equal too or greater than three “Gain = 3″ for oscillations to start because as we have seen above, the input is 1/3 of the output. This value, ( Av ≥ 3 ) is set by the feedback resistor network, R1 and R2 and for a non-inverting amplifier this is given as the ratio 1+(R1/R2). • Also, due to the open-loop gain limitations of operational amplifiers, frequencies above 1MHz are unachievable without the use of special high frequency op-amps.