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UNIT 2 OP-AMPS AS AC AMPLIFIERS

Subject Name: LINEAR IC’s AND APPLICATIONS Subject Code: 10EC46 Prepared By: Aparna.P Department: Electronics and Communication Date:29-3-2015. UNIT 2 OP-AMPS AS AC AMPLIFIERS. Introduction Capacitor Coupled Voltage Follower High Zin Capacitor Coupled Voltage Follower

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UNIT 2 OP-AMPS AS AC AMPLIFIERS

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  1. Subject Name: LINEAR IC’s AND APPLICATIONSSubject Code:10EC46Prepared By: Aparna.PDepartment: Electronics and CommunicationDate:29-3-2015

  2. UNIT 2OP-AMPS AS AC AMPLIFIERS

  3. Introduction • Capacitor Coupled Voltage Follower • High Zin Capacitor Coupled Voltage Follower • Capacitor Coupled Non inverting Amplifier • High Zin Capacitor Coupled Non inverting Amplifier. • Capacitor Coupled Inverting Amplifier • Setting the Upper Cutoff Frequency. • Difference Amplifier. • Use of a single Polarity Power Supply.

  4. Introduction • Op amps can be used as DC as well as AC amplifiers. • For op-amp AC amplifier the coupling capacitors are required at the input and out put. • The capacitors are not allowed to interrupt the bais current paths. • The capacitors have high impdance at low frequency. • The capacitor values are determined at FL.

  5. Capacitor Coupled Voltage Follower • The capacitor coupled voltage follower is shown in fig 1. The non inverting input terminal is connected to ground through R1. This is required to pass bias current to the op-amp non-inverting terminal. • The output capacitor blocks the DC offset output voltage. • The design of capacitor coupled voltage follower includes finding the R1,C1and C2. • The resistor values are selected as maximum values to minimize the power dissipation and minimum current drawn from the power supply. • The maximum value for resistor R1 is calculated from (0.1VBE)/ IB

  6. Capacitor Coupled Voltage Follower • The circuit input impedance is R1// Zi’, where Zi’ =[Zi(1+Mβ)] is the input impedance at the op-amp non-inverting input terminal. • But Zi’ is very large compared to R1. Then the input impedance is Zin = R1 • At lower 3dB frequency, the impedance of C1 should be small than Zin, so that no input signal will be across Xc1.

  7. Capacitor Coupled Voltage Follower • The capacitor C1 can be calculated as • The output is divided across Xc2 and RL is When Xc2 = RL

  8. High Zin Capacitor Coupled Voltage Follower • The input impedance of the capacitor coupled amplifier is very low . • The input impedance can be increased by connecting a capacitor C2 which couples the circuit output voltage to the junction of resistors R1 and R2. • C2 behaves as an Ac short circuit so that Vo appears across R2. • The voltage developed across R1 is V1 = Vs- Vo = Vs - MV1 • V1(1+M) = Vs

  9. High Zin Capacitor Coupled Voltage Follower • The input current is given by i1= V1/R1= Vs / (1+M)R1 • The input resistance is given by Zin = Vs / i1 Zin = (1+M) R1 • To design high input impedance capacitor coupled voltage follower, resistors R1 and R2 are calculated by assuming as single resistor R1max. Then it is divided into two equal resistors R1 and R2. • C1 can be calculated same as in the capacitor coupled amplifier. • The capacitor C2 can be calculated by Xc2 = R2 /10 at FL C2 = 1/ 2πf1(R2/10) • C3 can be calculated as Xc3 = RL C3 = 1/2πf1RL Here R1 is equal to R2 we will get C1 = C2

  10. Capacitor Coupled Non inverting Amplifier • In this capacitor coupled non inverting amplifier the non inverting input terminal is connected to ground through the resistor R1. • R1 ,C1 and C2 are calculated same as capacitor coupled voltage follower. • R2 and R3 can be calculated same as non inverting amplifier. • R3 = Vi/ I2 and AV= R2+R3 / R3 From this we can calculate R3

  11. High Zin Capacitor Coupled Non inverting Amplifier • In the capacitor coupled non inverting amplifier the input impedance is very low. • In order to increase the input impedance we are connecting C2. • The feedback factor β = R3/(R2 +R3) Zin = (1+Mβ)R1 Xc2 =R3 C2= 1/ (2πf1R3) • The other two capacitors can be calculated same as voltage follower.

  12. Capacitor Coupled Inverting Amplifier • In this the base current to the op-amp inverting input terminal flows via resistor R2 • The resistor values can be calculated same as direct coupled inverting amplifier. • The C1 and C2 can be calculated same as in non inverting amplifier.

  13. Setting Upper cutoff Frequency • The high frequency that can be processed by an op amp depends on the op-amp selected. • In some cases ,the upper cutoff frequency is higher than the desired. • This can be achieved by connecting a feed back capacitor. The gain is given by

  14. Setting Upper cutoff Frequency • Assume Rf= R2

  15. Capacitor Coupled Difference Amplifier • The resistor values can be calculated same as direct coupled difference amplifier. • The capacitors can be calculated as

  16. Single polarity voltage follower • The capacitor coupled voltage follower with single supply is shown in fig. • The potential divider sets the bias voltage at the non inverting input terminal as VCC/2. • The potential divider resistors are determined by choosing resistor current very much larger than the op-amp input bias current. • The voltage drop across each resistor is VCC/2.

  17. Single polarity voltage follower • The input impedance of the circuit is given by Zin = R1// R2 XC1= (R1// R2)/10 at f1 And Xc2 = RL at f1 The input impedance of this circuit is very less it can be increased by connecting the capacitor C2.

  18. Single polarity High input impedance voltage follower • With this Circuit the input impedance can be increased. • Finding the resistor values is same as voltage follower. • And the capacitor values can be calculated as Xc1= R3/10 at f1 Xc2 = (R1// R2)/10 at f1 Xc3 = RL at f1

  19. Single polarity Non Inverting Amplifier • The potential divider biases the op-amp non inverting input terminal at VCC/2 • For AC signals C3 act as Short circuit. • The values of C1,C2,R1,R2 can be calculated same as Voltage follower. • C3 is chosen so that the reactance is less than the R4. • Xc3=R4/10.

  20. Single polarity High Zin Non Inverting Amplifier • It is similar to the high Zin voltage follower. • Here R4 is connected in the non inverting amplifier to provide the gain. • To design the circuit R1 and R2 can be calculated same as voltage follower. • The capacitor values can be calculated same as non inverting amplifier.

  21. Single polarity Inverting Amplifier • The inverting amplifier with single polarity is shown in fig. • The potential divider is used to provide biasing voltage at the non inverting input terminal as VCC /2 . • The same voltage appear at the inverting terminal also. • R3=R4 = (Vcc/2 ) / I4 • The Remaining parameters can be calculated same as capacitor coupled inverting amplifier.

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