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Basic amplifier concepts Voltage gain reducing by 1/2 times indicates that power will be reduced by half, at the half power frequencies. 1 Basic amplifier concepts However, often the signal contain a “common” voltage applied to both terminals that also affects the output signal. This is not desirable, and a good amplifier needs to minimize such effects. 2 Basic amplifier concepts The patient has a common mode input due to the parasitic capacitances. The ECG machine must be able to ignore the common mode signal and not amplify it. 3 Basic amplifier concepts 4 Basic amplifier concepts 5 Basic amplifier concepts 1. Measure the common mode gain Acm 2. Measure the differential gain Ad 3. Compute the CMRR = 20 log Ad/Acm 6 Problems P1.17: An FM radio receiver has an input resistance of 75 Ohm. The input signal provided by a distant transmitter is 5 µV rms, and the receiver supplies 5 V rms to an 8 Ω loudspeaker. Calculate the power gain of the receiver. P1.33: An amplifier has a voltage gain of 30 dB, and a current gain of 70 dB. What is the power gain in dB? If the input resistance is 100 KΩ, what is the load resistance? P1.63: A differential amplifier has vo (t) = A1v1(t) – A2v2(t), If A1 = 1000, and A2 = 999, determine the CMRR. Answers: P1.17: 9.375 x 1E12 P1.33: GdB = 50, RL = 1 KΩ P1.63: CMRR = 60 dB Answer: -20 dB and CMRR = 114 dB 7