Subject Name LINEAR ICs AND APPLICATIONS Subject Code

Subject Name: LINEAR IC’s AND APPLICATIONS Subject Code: 10 EC 46 Prepared By: Aparna. P Department: Electronics and Communication Date: 29 -3 -2015 11/24/2020

UNIT 2 OP-AMPS AS AC AMPLIFIERS

• • • 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.

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.

Capacitor Coupled Voltage Follower • The capacitor coupled voltage follower is shown in fig 1. The non inverting input terminal is connected to ground through R 1. 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 R 1, C 1 and C 2. • 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 R 1 is calculated from (0. 1 VBE) / IB

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

Capacitor Coupled Voltage Follower • The capacitor C 1 can be calculated as • The output is divided across Xc 2 and RL is When Xc 2 = RL

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 C 2 which couples the circuit output voltage to the junction of resistors R 1 and R 2. • C 2 behaves as an Ac short circuit so that Vo appears across R 2. • The voltage developed across R 1 is V 1 = Vs- Vo = Vs - MV 1 • V 1(1+M) = Vs

High Zin Capacitor Coupled Voltage Follower • The input current is given by i 1= V 1/R 1= Vs / (1+M)R 1 • The input resistance is given by Zin = Vs / i 1 Zin = (1+M) R 1 • To design high input impedance capacitor coupled voltage follower, resistors R 1 and R 2 are calculated by assuming as single resistor R 1 max. Then it is divided into two equal resistors R 1 and R 2. • C 1 can be calculated same as in the capacitor coupled amplifier. • The capacitor C 2 can be calculated by Xc 2 = R 2 /10 at FL C 2 = 1/ 2πf 1(R 2/10) • C 3 can be calculated as Xc 3 = RL C 3 = 1/2πf 1 RL Here R 1 is equal to R 2 we will get C 1 = C 2

Capacitor Coupled Non inverting Amplifier • In this capacitor coupled non inverting amplifier the non inverting input terminal is connected to ground through the resistor R 1. • R 1 , C 1 and C 2 are calculated same as capacitor coupled voltage follower. • R 2 and R 3 can be calculated same as non inverting amplifier. • R 3 = Vi/ I 2 and AV= R 2+R 3 / R 3 From this we can calculate R 3

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 C 2. • The feedback factor β = R 3/(R 2 +R 3) Zin = (1+Mβ)R 1 Xc 2 =R 3 C 2= 1/ (2πf 1 R 3) • The other two capacitors can be calculated same as voltage follower.

Capacitor Coupled Inverting Amplifier • In this the base current to the op-amp inverting input terminal flows via resistor R 2 • The resistor values can be calculated same as direct coupled inverting amplifier. • The C 1 and C 2 can be calculated same as in non inverting amplifier.

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

Setting Upper cutoff Frequency • Assume Rf= R 2

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

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.

Single polarity voltage follower • The input impedance of the circuit is given by Zin = R 1// R 2 XC 1= (R 1// R 2)/10 at f 1 And Xc 2 = RL at f 1 The input impedance of this circuit is very less it can be increased by connecting the capacitor C 2.

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 Xc 1= R 3/10 at f 1 Xc 2 = (R 1// R 2)/10 at f 1 Xc 3 = RL at f 1

Single polarity Non Inverting Amplifier • The potential divider biases the op-amp non inverting input terminal at VCC/2 • For AC signals C 3 act as Short circuit. • The values of C 1, C 2, R 1, R 2 can be calculated same as Voltage follower. • C 3 is chosen so that the reactance is less than the R 4. • Xc 3=R 4/10.

Single polarity High Zin Non Inverting Amplifier • It is similar to the high Zin voltage follower. • Here R 4 is connected in the non inverting amplifier to provide the gain. • To design the circuit R 1 and R 2 can be calculated same as voltage follower. • The capacitor values can be calculated same as non inverting amplifier.

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. • R 3=R 4 = (Vcc/2 ) / I 4 • The Remaining parameters can be calculated same as capacitor coupled inverting amplifier.