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Old 29th April 2003, 05:23 AM   #41
sajti is offline sajti  Hungary
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Join Date: Apr 2003
Location: Vác, Hungary
Quote:
Originally posted by moamps


Hi,

Voltage Vb of Q2 and Q3 is a little bit under 0V. Because of that, C4 is connected with +pol to ground. In dynamic conditions maximum value of Vce for these transistors can be +rail + ca 1V, I think. At least in theory.

But, what happens during a short time interval between powering on or off and the steady state condition? Or, what happens when you have short static charge in input (in PA systems), or if you get RF garbage voltage modulated on +rail?

My experience has taught me that input BJT-s must have Vceo about +/-Vps, especially because we have good and inexpensive BJT-s for this (for example 2sa970, 2sc2240, 2n5401, 2n5551). IMHO

Regards

Hi,

I agree with the first part of Your post. The base of the input device is connected to the ground.
What happend when we switch ON the amplifier?
1, Positive rail wake up first. There is no current from the tail. Transistors are switched off, base at 0V, collector at +rail.
2, Negativ rail first. There is no positive rail the collectors at 0V, base little less voltage due the emitter current.

In the PA amplifier the first stage is not the power amplifier input device. There is the balanced-unbalanced converter, or there is the limiter, or active bridgeing switch. But if You want to protect Your amplifier just use simple series resistor, and zeners to limit the input voltage.

For +rail RF modulation: If You use proper filtering, and good quality capacitors there is not possible to get 100% modulation. Few % is possible.

So my experience is: If You apply input device with 20% higher Vcemax, than Your positive rail, there will be no problem.

Sajti
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