How the Vbe multiplier works in a push-pull amp?

[sec]

Can someone help me understanding how the Vbe multiplier work in a push-pull amp. And how the value of the resistor are chosen ?
thanks !!

 

[diegobb]

vbe multiplier circuit

The Vbe multiplier is used to avoid the dead zone in the output stage of an amplifier.
The problem is that due to the Vbe of the two transistors in the output stage, there is a dead zone, and for some reason's you don't want tihs.
To avoid this the idea is to put an aditional 2*Vbe betwen the bases of the output stage transistor, this is made with a Vbe multiplier.

To choose the values you first must ensure that an enought current is provided to the Vbe multiplier by the polarization.
Supose you are using an NPN transistor with a Vbe(on) = Vbe, and two resistors, R1 and R2, where R1 is connected betwen the base and the colector; and R2 is connected betwen the base and the emiter.
The current flowing thru R2 is I_R2=Vbe/R2, so the voltage in R1 is V_R1=I_R2*R1 (Asumming Ib<<I_R1 and I_R2), in this case you obtain a total voltage betwen the emiter and the colector of the transistor of Vce = V_R1+Vbe where V_R1=(Vbe*R1)/R2, so: Vce = Vbe + Vbe*(R1/R2).

I hope this can help you, and excuse my not so good english.
Bye and gook luck

 

[A.Anand Srinivasan]

vbe muliplier

Vbe multiplier is made used to prevent crossover distortion... but using diode doesnt work perfectly well due to the mismatch between the base emitter diode and the other diode used and hence a small resistor is connected at the emitter that is all i've heard of.....

 

[diegobb]

vbe multiplier

Mmmm, if i'm not wrong (And i think i'm not) the small resistor in the emiter is used by the current overflow stage to monitor the current flowing thru the output stage. Usually this resistor is betwen 0.5 and 5 ohms, or so.
If the voltage sensed in this small resistor goes over a desired value (So the current in load goes over a desired value) , the output stage goes down.

 

[A.Anand Srinivasan]

vbe multiplier output

ya... you are right.. that is also one of the application of that resistor.... the other use is to provide a natural curve in the crossover region....

 

[sec]

vbe multiplyier

so the vbe multiplier can be a sort of current limiter!?

Added after 5 hours 1 minutes:

Can it been done with a PNP

 

[A.Anand Srinivasan]

vbe multiplier problem

i dont what you are trying to ask..... a push pull already has a pnp involved.... please post the circuit you have in mind involving the PNP....