MUTING_CIRCUIT

Published:2009/6/23 2:43:00 Author:May | From:SeekIC

The circuit operates as follows: The signal is input to Vin and a dc control voltage is applied to VC. Vbias determines the desired bias point current. Assuming the following component and voltage values: VCC＝7.6 VdcVbias＝1 VdcVin＝1Vp-p，ac signal centered about 3.8 Vdc biasRL＝2kΩRE＝200kΩRE1＝2kΩ Q3 bias current is 1 mA, and dc output voltage is about 5.8Vwith an ac gain of about -1, Q1 and Q2 form a current switch and Q3 acts as a constant current source.For unmuted operation, Vc=0Vdc, and all of the bias current flows through Q1. Consequently, the circuit operates as a normal common emitter stage, with ac gain=-RL/RE1,. When Vc=5Vdc, all of the bias current flows through Q2, reducing the signal gain to zero. However, because the same dc current flows through RL, in both cases(unmuted and muted), the bias point at the output remains fixed. The Cc/RE1network is required to bypass the Q3 current source(which is a high impedance)to achieve alow ac impedance at the emitter of the Q1 common emitter stage during unmuted oper-ation. Cc is chosen to be a short circuit at signal frequencies of interest. The circuit works best if the Q1 and Q2 pair is matched. Typical change in the output dc voltage from unmuted to muted condi-tion is <5 mVdc. RL,RE, and Vbias are chosen for desired dc operating conditions and signal dynamic range. Vbias, can be generated via a VCC voltage divider. The signal at Vin, can be ac coupled, but a bias circuit must be added to Q1's base to generate a dc component. RE1 is chosen for desired ac gain. Vin must be cen-tered about a dc component, and, to assure proper switching action, VCTRL, must be higher than Vin, by an amount greater than one VBE drop.



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