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Skorpio 7th May 2013 10:55 AM

Modifying Emitter Follower type I

A am currently modifying an old amp and the driver/output is a type I emitterfollower. As type II has better performance, I want to upgrade.

The two driver resistors are 220R. I want to lift them from output and connect them in series (440R total). Will this affect stability or other things?

I see also that there are different views on decoupling the resistor:

D. Self:
5.3.3 Switchoff distortion. (Distortion 3c)
The EF Type II configuration in Fig 13b is at first sight merely a pointless variation on Type I, but its valuable property is that the shared driver emitter-resistor Rd, with no output-rail connection, allows the drivers to reverse-bias the base-emitter junction of the output device being turned off..
Speed-up capacitor Cs improves this action, preventing the charge-suckout rate being limited by the resistance of Rd. A 1 uF speed-up capacitor can half the THD at 40kHz, implying cleaner switchoff.

The second influence on turnoff is the value of the driver emitter or collector resistors; the lower they are the faster the stored charge can be removed. Applying these two criteria can reduce HF distortion markedly, but of equal importance is that it minimises overlap of output conduction at HF, which if unchecked gives an inefficient and potentially destructive increase in supply current.
Distortion In Power Amplifiers

Fig. 8(b) shows a simplified diagram of the amplifier output stage (the driver transistors are omitted) with a capacitor shown in parallel with R36. Because the VBE multiplier holds the voltage across R36 constant, the addition of the capacitor has no effect on the voltages at the bases of Q18 and Q19. If speedup caps are necessary, they should be added in parallel with R41 and R42. However, these 10 ohm resistors are too small for the speedup caps to have any effect
The Leach Amp - Output Stage

What would You recommend?


orbanp 8th May 2013 04:46 PM

You did not mention what power is your amplifier. Upgrade the final configuration, though most probably you will not notice much difference.
If anything changes, it could be high frequency parasitic oscillations at switchover, at least that I found when I simulated various output configurations. The speed up capacitor helps quenching the oscillations.
The other change might be different final bias. When bringing up your new circuit, start from zero final bias current and increase it to the desired value. Previous setting might be too high!
Probably best advice is to plug your modified circuit into the Spice simulator and see what effect the various values would have!
The Leach amp circuit is different, it uses a CFP configuration (Sziklay pair), that stage itself could be oscillating. The resistors series to the base of the final transistors are to prevent this.
The leach amp is also generally higher power than Self's blameless amp.

Good luck, Peter

Skorpio 10th May 2013 06:33 AM

Thanks for replying!

I changed the circuit some days ago, and no changes in bias or oscillations were present. It seems to have a small sonically20638616rovement, but not sure here at this stage...

Regarding Leach, look at figure 8.b and compare with Self

orbanp 10th May 2013 12:57 PM

You are correct, the Leach amp does not use CFP output stage.
Not sure what I was thinking...
(one should not believe everything one is reading on the internet ;-)

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