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The Common-Collector Amplifier The common-collector amplifier is usually referred to as the emitter follower because there is no phase inversion or voltage gain. The output is taken from the emitter. The common-collector amplifier’s main advantages are it’s high current gain and high input resistance. The Common-Collector Amplifier The input resistance can be determined by the simplified formula below. Rin(base) ≅ βac(r’e + Re) Rin(tot) = Rin(base)//R1 // R2 Output Resistance is very low: Rout ≅ RE (Rs /βac) The Common-Collector Amplifier The Voltage Gain can be determined by the simplified formula below. Av = Re/(r’e + Re) and is always less than 1 but if r’ e << Re then Av ≅ 1 is a good approximation. The current Gain: Ai ≅ βac with R1 // R2 >> Reβac The Power Gain: Ap = AvAi ≅ Ai with Av ≅ 1 Example Example Example 6.9: Calculate Input Resistance, Voltage Gain, Current Gain and Power Gain for the given circuit. DC analysis: VE = R2 /(R1+R2 ) - VBE = 4.3V DC analysis: IE = VE /RE = 4.3mA AC Analysis: r’e =25mV/IE = 5.8 Example Example 6.9: Calculate Input Resistance, Voltage Gain, Current Gain and Power Gain for the given circuit. Re = RE // RL : 1k//1k = 500 Rin(base) ≅ βac(r’e + Re) but r’e is just 5.8 so Rin(base) ≅ βac(Re) = 175(500) = 87.5k Rin(tot) = Rin(base)//R1 // R2 = 8.16k Example Example 6.9: Calculate Input Resistance, Voltage Gain, Current Gain and Power Gain for the given circuit. Av = Re/(r’e + Re) = 500/505.8 = 0.989 Ai = Ie/Iin = (Ve/Re)/(Vin/Rin(tot))= = (VbAv/500)/(1/8.16k) = 2mA/123A = 16.3 Conclusion The output resistance is very low. This makes it useful for driving low impedance loads. The current gain(Ai) is approximately βac. The voltage gain is approximately 1. The power gain is approximately equal to the current gain(Ai). The Common-Collector Amplifier The darlington pair is used to boost the input impedance to reduce loading of high output impedance circuits. The collectors are joined together and the emitter of the input transistor is connected to the base of the output transistor. The input impedance can be determined the formula below. Rin = βac1βac2Re Tutorial and Assignment • Solve Example 6.9 and 6.10 and send to me on Wednesday before class. • Tutorial II : Qs 18 to 23 pages 319-320. To be discussed on Thursday. The Common-Collector Amplifier