# Class B amplifiers with symmetrical power supply

Discussion in 'Homework Help' started by jboavida, Jul 8, 2009.

1. ### jboavida Thread Starter Member

Jul 10, 2008
23
0
Hi,

How do we perform the calculations on a class B amplifier with simmetrical power supply for instance (+15V and -15V).

Do we consider vcc = 30V?

For instance if we have vcc=15V

The quescient current will be 15*2VBE / 2R

if simmetrical PS will be 30*2VBE / 2R ?

And the other calculations like power that also include vcc?

See attached schematic please

Thanks

JB

File size:
22.5 KB
Views:
39
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,667
1,311
If we ignore the base current, then current that will flow through R1 and R2 and D1,D2 will be equal:
I=(30V-1.4V)/15K=1.9mA
And what power do you want to know?

3. ### jboavida Thread Starter Member

Jul 10, 2008
23
0
Pout (Maximum output power considering 8 ohm load)
and power delivered by the power supply.

Both use vcc to calculate.

Thanks for your anwser.

JB

4. ### millwood Guest

assuming no loss on the transistors (typically 1-2v for bjts), the peak output on the load will be Vcc.

so the power output on a load is Vcc^2/(2*Rl) rms where Rl is the load.

instantenous power output (a poorly defined term and sometimes refereed to as "Mamximum power output" on some boomboxes is Vcc^2/Rl. or 2x its rms rating.

5. ### millwood Guest

the ideal current is undefined in the circuit.

6. ### jboavida Thread Starter Member

Jul 10, 2008
23
0
Yes but we consider vcc 15V or 30v?

Thanks

7. ### millwood Guest

an easier way to think about this all is to look at it from the power supply's perspective. the positive power supply is outputting 15v Vcc and an idle current of 2ma (for example). so 15v*2ma=30mw of power is provided to the amp/load by the positive power supply.

do the same math with the negative power supply and you get 30mw as well. so total power consumption between the amp / load is 30mw+30mw=60mw.

this thought process will work regardless of how the rail is configured.