TRANSISTOR_TURNS_OP_AMP_ON_OR_OFF

Published:2009/7/10 21:22:00 Author:May | From:SeekIC

When transistor Q1 is switched off, the circuit behaves as a voltage follower. By applying a positive voltage to the emitter of Q1 via a 10 Ω resistor, the transistor is made to turn on and go into saturation. Thus, the lower end of R4 is connected to ground. The circuit has not changed into that of a differential amplifier, except that the voltage difference is always 0 V As long as the resistor ratios in the two branches around the op amp are in the same ratio, the output should be zero. A 47-kΩ resistor is used to null out any ratio errors so that the OFF attenuation is more than 60 dB. The high common-mode rejection ratio of a 741 enables this large attenuation to be obtained.



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