transistor biasing circuits

Thread Starter


Joined Aug 8, 2005
can anyone tell me about bootstrap and emitter feedback biasing circuits? anything would be good to know like schematics, operation, other names, and formulas. thanks


Joined Jan 22, 2004

have you tried thisBJTs?

there are 2 ways to configure an emitter feedback transistor circuit.
a. having a single base bias resistor & having an emitter resistor bypass w/a cap
b. having a potential divider base bias & a split emitter resistor

item a. this configuration offers thermal stability due to the emitter resistor. as the temperature increase Ve likewise increases. as Ve increase Vbe also decrease, Ib also increase due to less forward bias, effectively decreasing Ic. the downside of this circuit is gain stability

item b. this configuration offers a much improved thermal stability and gain stability. base current is provided by the potential divider while the emitter resistor provides Ve that increases with temperature rise. the current drawn by the potential divider is in the range of about 10 times Ibq, w/c ensures that the potential at the potential divider junction remains stable even when Ibq changes slightly. so when temperature increases, Ie increases - Ic increases but Ib remains constant due to the potential divider. effectively, Vbe decreases hence Ib decreases w/c leads to a decrease in Ic. if you wish to further increase thermal performance, you can add a forward biased diode connected in series to R2 of the potential divider. another favorable characteristic feature of this configuration is improved gain stability. this is achieved by splitting the emitter resistor into Re1 + Re2. Re1 is used for dc biasing while Re2 is used for ac operation to allow the transistor diffusion resistance to vary when Ie >> Ic.