Q point for a BJT

[didi10]

I need a relationship (equation) between Cpi and the Q point for a BJT.

[sachinelectrostar]

Vbe - Vcc + (Ic + Ib)Rc + IbRb = 0

sinc Ic >> Ib we can simplify to

Vbe - Vcc + IcRc + IbRb = 0

because Ib = Ic/beta we can solve for Ic to get

Ic = (Vcc - Vbe) / (Rc + Rb/beta)

[dscpcb]

The Q, or quiescent point, is the DC level, or where the circuit produces no noise.
The first thing to do would be to Thevenize the left side of the circuit; Vth = 24K/(24K+96K) * Vcc, Rth = 96K in parallel with 24K.

Once you solve those, you will have a circuit with an a voltage source on the left (Vth) and a resistor, Rth, between Vth and Vbe.

To solve for the current i(b), use i(b) = (Vth - Vbe)/Rth

Once you have i(b), you should be able to find i(c) and Vce, and then verify that the circuit is in active mode.

[bwetman]

Hello,
The operating point of a BJT, also known as bias point or quiescent point (or simply Q-point), is the DC voltage and/or current which, when applied to a device, causes it to operate in a certain desired fashion.