don't know how to estimate the dc voltage at collector of q4

[ashade]

hi
i am an engineering electronics student and i was trying to design my own power amp. It's not about to be tested on a protoboard... it's just for simulating...
well, i'm having trouble with dc levels because i don't know how to estimate the dc voltage at collector of q4. Q4 and Q3 form a bjt differential pair activelly loaded with pnp transistors q6 and q5. q11 is an emitter follower, so it doesn't load the q4/q3 state.
Here's how i am calculating dc voltages and currents:

1) assuming the two inputs are zero, voltage at bases of q1 and q2 are zero, voltages at base of q3/q4 are -0.7V;
2) since Vcc=+25V, base of q5 is at 24.3V and, therefore, q3 collector is at 24.3V.
3) The problem: what is the voltage at collector of q4/q6???

**broken link removed**

 

[Miguel Gaspar]

Re: don't know how to estimate the dc voltage at collector o

According microelectronics datasheets:

http://www.datasheetcatalog.com/datasheets_pdf/B/C/5/5/BC559.shtml

first figure on page 4 for a Ic= 1mA, and Ib= 3µA, Vce = 5V

so Vc q4 = 25 - 5V = 5 Volts

 

[ashade]

Re: don't know how to estimate the dc voltage at collector o

alright, but according the simulation the value of dc at colector of q4 is 22V!!
oh sorry!!! 25-5 = 20V!! that's the correct value... thakx!

Added after 8 minutes:

fine. now, as u can see, the "ideal dc voltage" would be about 13V to allow maximum swing, but it's 21V... do u have any suggestions so i can change the circuit to match that dc voltage????

 

[Miguel Gaspar]

if you reduce Ic for example to 2.5uA that will give you aprox Vce aprox 13v

Added after 11 minutes:

sorry not ic, ib to 2.5uA
that reduces ic so at 1mA that gives you Vce aprox 12 V

 

[ashade]

Re: don't know how to estimate the dc voltage at collector o

how do u calculated that?? with the datasheet??? Vce goes only from 0 - 5V...
and how will i change ib without changing ic??? I thought in active mode ib = ic/(B+1)...

 

[Miguel Gaspar]

use the first reference that I gave you
from there you can see the Ic vs VCE you can extrapolate

I tryied to find The early voltage or a way to find out that, but I culdn't

SO I used a graphic method.

 

[ashade]

Re: don't know how to estimate the dc voltage at collector o

well, i made a try-error solution using the spice simulation, and the bias Ic must be equal to 25uA to achieve a dc voltage of 13V. however, it's a too low bias current, so it's not a good solution..