Wednesday 18 November 2015

circuit analysis - Differential stage Av value with current sources


Why all books told that Av differential of an differential input stage in power amp is very big? Ad = Rc/re, but if there are current sources in the emitter and collector circuit they will have too big impedance and division one big "Rc" to another big "re" will not give very big "Ad" value. Where is a trick?


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Answer




In the differential stage with asymmetric output, the load current can "swing" from:


\$0A\$ to \$I_{EE}\$


enter image description here


And the gain is


$$A_d = 0.5*R_C \approx \frac{R_C}{2r_e}$$


But by adding the current mirror we can "increase" the gain (the gain is now the same as it is for a symmetrical output). Because now the load current can swing from:


\$-I_{EE}\$ to \$+I_{EE}\$


enter image description here


And the gain is:


$$A_d = gm*R_L \approx \frac{R_L}{r_e}$$



In your audio amplifier the dif. stage the load resistance is equal to the input impedance of a VAS stage (Q6). Hence, the DC gain is "low" because \$r_\pi\$ is low and the Miller capacitance. http://www.ecircuitcenter.com/Circuits_Audio_Amp/Miller_Integrator/Miller_Integrator.htm


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