Example - 1
The following specifications are given for the dual input, balanced-output differential amplifier: RC = 2.2 kΩ, RB = 4.7 kΩ, Rin 1 = Rin 2 = 50Ω, +VCC= 10V, -VEE = -10 V, βdc =100 and VBE = 0.715V.
- Determine the voltage gain.
- Determine the input resistance
- Determine the output resistance.
Solution:
(a). The parameters of the amplifiers are same as discussed in example-1 of lecture-1. The operating point of the two transistors obtained in lecture-1 are given below
ICQ = 0.988 mA
VCEQ=8.54VThe ac emitter resistance
Therefore, substituting the known values in voltage gain equation (E-2), we obtain
b). The input resistance seen from each input source is given by (E-3) and (E-4):
(c) The output resistance seen looking back into the circuit from each of the two output terminals is given by (E-5)
Ro1 = Ro2 = 2.2 k Ω
Example - 2
For the dual input, balanced output differential amplifier of Example-1:
- Determine the output voltage (vo) if vin 1 = 50mV peak to peak (pp) at 1 kHz and vin 2 = 20 mV pp at 1 kHz.
- What is the maximum peal to peak output voltage without clipping?
Solution:
(a) In Example-1 we have determined the voltage gain of the dual input, balanced output differential amplifier. Substituting this voltage gain (Ad = 86.96) and given values of input voltages in (E-1), we get
(b) Note that in case of dual input, balanced output difference amplifier, the output voltage vo is measured across the collector. Therefore, to calculate the maximum peak to peak output voltage, we need to determine the voltage drop across each collector resistor:
Substituting IC = ICQ = 0.988 mA, we get
This means that the maximum change in voltage across each collector resistor is ± 2.17 (ideally) or 4.34 VPP. In other words, the maximum peak to peak output voltage with out clipping is (2) (4.34) = 8.68 VPP.
