Hi Mr. Cordell
I am reading page 196 regarding the drive requirement of the power BJT. Equation 10.1 C\pi =gm/(2\pi fT) is hybrid pi in common emitter configuration. In the output stage, the transistor is in common collector configuration. Should the derivation be different?
I am not familiar with driving low impedance load. I have look around a little in high frequency input impedance of common collector stage and have no luck. But it seems like gm should be a lot lower. In push pull stage, the emitter of the complementary transistor is following the same voltage swing so the low output impedance of the complementary transistor has not effect the calculation. If you use 50mA idle current and RE=0.5 ohm, the impedance see by the emitter of the transistor is RE+speaker = 4 ohm( about). Should that lower the gm and should C\pi be a lot smaller? Can you explain a little on this?
Thanks
I am reading page 196 regarding the drive requirement of the power BJT. Equation 10.1 C\pi =gm/(2\pi fT) is hybrid pi in common emitter configuration. In the output stage, the transistor is in common collector configuration. Should the derivation be different?
I am not familiar with driving low impedance load. I have look around a little in high frequency input impedance of common collector stage and have no luck. But it seems like gm should be a lot lower. In push pull stage, the emitter of the complementary transistor is following the same voltage swing so the low output impedance of the complementary transistor has not effect the calculation. If you use 50mA idle current and RE=0.5 ohm, the impedance see by the emitter of the transistor is RE+speaker = 4 ohm( about). Should that lower the gm and should C\pi be a lot smaller? Can you explain a little on this?
Thanks
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No, Cpi=Cb'e is not depending on the device circuit (common emitter or common collector). It's a parameter model (actually a fair approximation, for the normal bipolar operation, of a much more complex physical reality - as all the other hybrid pi parameters).
Yes, but Cbe is no longer grounded at the emitter side, so the circuit is different. If r/pi is Beta/gm, then the equivalent resistor seen at the input due to Rload is (Beta X Rload). In this case, Rload = RE + Speaker = 4 ohm. Assume Beta =50, so the equivalent at the input is 50 X 4 = 200ohm in series with the Cbe.
In another way, if the input still have the same Cbe to the ground, The VAS has to provide current to charge the Cbe. Then the use of EF would be very limited as during dynamic swing, it's the Cbe that load down the VAS not the current drive alone.
That's why I would like to see the derivation of the dynamic drive requirement. I have no luck looking in the books I have or on web. They are just talking about simple low frequency model without the Cbe influence.
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Hi Alan,
Sorry for the confusion. There are simplified explanations that can provide insight, but it is true that the hybrid pi model itself is a simplification. Indeed, and most importantly, the hybrid pi is a small-signal model. Most of the hybrid pi parameters change with the current and voltage conditions of the transistor at the moment. Most importantly transconductance (gm) is proportional to Ic in an ideal transistor. If ft were constant, then Cpi would be proportional to Ic. For insight and simplification when we are waiving our hands, we assume that ft and beta are constant, but in the real world they change somewhat to current and voltage conditions.
Keep in mind that Cbe or Cpi is connected from base to emitter, not from base to ground. In an emitter follower, Cbe is bootstrapped by the output signal, so the input impedance attributable to Cbe is smaller - just like as with Rpi.
For some, it is an eye-opener when they calculate Cpi for a power transistor operating at 1 amp and having an ft of 10MHz. It is a non-trivial amount of capacitance.
For VAS loading from a small-signal point of view, the capacitive component of the output transistor will be VERY roughly Ccb + Cbe*(1-G), where G is the voltage gain of the emitter follower. But this is an extreme simplification that involves a bit of hand-waiving. A different rough approximation for the effective capacitive impedance seen due to Cbe is ac beta times the load resistance, recognizing that ac beta is falling with increased frequency at high frequencies.
Hope this helps a little.
Cheers,
Bob
Thanks Mr. Cordell.
Sorry I did not respond back until today, I was busy with my guitar amp trying to finish up and start on the audiophile power amp.
It makes a lot more sense that driving an emitter follower does not see the full Cpi but seeing about Cpi(1-G) which is a lot smaller.
Thanks
Sorry I did not respond back until today, I was busy with my guitar amp trying to finish up and start on the audiophile power amp.
It makes a lot more sense that driving an emitter follower does not see the full Cpi but seeing about Cpi(1-G) which is a lot smaller.
Thanks
Amplifier Stability question
Hello Bob,
I have a question on power amplifier stability. I have an amplifier which has a good square wave response into 4 ohm load with a 1uF cap in parallel you have some ringing which is low in amplitute an ceases very quickly.
Now when I reduce the capacitance to 0.47uf to 0.22uf to 0.1uf the transient square wave respone improves progressively as the capacitance decreases to a point where the transient response does not appear to be affected by the load. I then drop the capacitor to 10nf I find what appears to be a self sustaining oscillation appears super imposed on the square wave , and if I drop the value to 1nf (1000pf) the amplfier goes into a full blown oscillation 3.5Mhz which is large in amplitute ( can blow the amp up).
My opinon is that I have some parasitic oscillation affect some where but I was interested in your opinion, have you had similar experiences an if so how did you fix it.
Regards
Arthur
Hello Bob,
I have a question on power amplifier stability. I have an amplifier which has a good square wave response into 4 ohm load with a 1uF cap in parallel you have some ringing which is low in amplitute an ceases very quickly.
Now when I reduce the capacitance to 0.47uf to 0.22uf to 0.1uf the transient square wave respone improves progressively as the capacitance decreases to a point where the transient response does not appear to be affected by the load. I then drop the capacitor to 10nf I find what appears to be a self sustaining oscillation appears super imposed on the square wave , and if I drop the value to 1nf (1000pf) the amplfier goes into a full blown oscillation 3.5Mhz which is large in amplitute ( can blow the amp up).
My opinon is that I have some parasitic oscillation affect some where but I was interested in your opinion, have you had similar experiences an if so how did you fix it.
Regards
Arthur
This can be normal behavior. Every good amplifier has output inductance, so ringing with a cap on the output is just a sign that it's working right. Some amplifiers overdo it and capacitive loads will actually oscillate. Often it does occur in the 1nF range.
Try adjusting the L//R at the output and see if that works against the oscillation. If you can't fix it with the L//R, then I would start looking into parasitics and feedback instability.
Try adjusting the L//R at the output and see if that works against the oscillation. If you can't fix it with the L//R, then I would start looking into parasitics and feedback instability.
Hi Arthur,
I'm not surprised at all. Many of my simulations show the same phenomenon. Most amps (w/o Zobel) don't like caps between 5...50nF or so. If your amp already starts oscillating with only 1nF, it's clearly a sign of marginal stability. You could hide this lack of stability by means of a Zobel network, of course. But wouldn't that be a stopgap?
I also like to hear what Bob has to say about it.
Cheers, E.
PS: What about a spelling checker?
Hi Keane,
If it starts oscillating with only 1nF, I think there isn't a Zobel network at all.
Cheers, E.
and the short cable inductance to the dummy load is isolating the dummy load from the amplifier output at VHF.
The cable capacitance has become the only load at VHF and it just needs a tiny kick to set up full oscillation.
Adding in a VHF load between the amplifier OUTPUT and the amplifier HF Decoupling may be enough to prevent the amplifier gain going haywire at VHF.
i.e. adding an output Zobel in the correct location may be all it needs.
That's a pretty normal behavior (short of the low 1nF that triggers the oscillations).
A large load capacitance may be seen as a shunt frequency compensation, the same way as a voltage regulator is compensated by the output cap (and usually requires an electrolytic with rather large ESR and ESL to remain stable). Such a large cap is limiting the amplifier ULGF, effectively moving the dominant pole at the output cap. As such, the phase stabilility condition is largely met, and all you get is the amortized transient response of the output cap and the inherently inductive output impedance in any global feedback amplifier.
Now, when the output cap value is decreased to some 10-100nF, this no longer creates a dominant pole, but just another pole that falls in before the ULGF. As such, the gain drops at 12dB/oct and the phase may go beyond the stability condition. Please note that this output pole is NOT splitted by the negative feedback network, because it is NOT in the feedback loop!
This would explain why most amplifiers don't like a 10-100nF cap at the output, being in fact much more tolerant to 1-2uF. The open loop output impedance plays also an important role in this behavior.
It would though not explain why your amp starts oscillating with 1nF of output cap. I would think that your amp has an unusually high ULGF, and it does not properly separate/split the open loop poles. Or the gain margin is to low at HF. As such, another pole at HF is just enough to trigger a local or global oscillation condition. One to another, you should review your amp frequency response around and beyond the ULGF and see how a 1-10nF affects it. This could be difficult to simulate, given the current devices stock models validity.
For most amps, with the usual Zobel and Thiele networks, 1 - 10 nF is most likely load to excite minor or full blown oscillation.
This is true both for SPICE and real world.
WHY IS THIS IMPORTANT?
Instability with small capacitance correlates well with instability with real speakers on part of the cycle and dependent on the thermal and signal history of the amp. Try testing with a big guitar speaker at various power levels and frequencies.
Many Golden Pinnae designs, including by some famous names that post on this forum, have this evil
Zobel, Thiele, etc... are not included in this discussion. They are there precisely for isolating the output from capacitive loads, but I would not buy an amp that relies only on these for avoiding bursting into oscillation from a 10nF cap load.
For most amps, with the usual Zobel and Thiele networks, 1 - 10 nF is most likely load to excite minor or full blown oscillation.
This is true both for SPICE and real world.
WHY IS THIS IMPORTANT?
Instability with small capacitance correlates well with instability with real speakers on part of the cycle and dependent on the thermal and signal history of the amp. Try testing with a big guitar speaker at various power levels and frequencies.
Many Golden Pinnae designs, including by some famous names that post on this forum, have this evil
Amplifier output zoble network and inductor used
Hello Guys
Thanks for the interest on my problem .
This is the network that I use on the output of my amplifier. There is a zoble network on the amplifier ground plane and the output inductor is on the rear panel of the amplifier away from amplifiers output stage.
Regards
Arthur
Hello Guys
Thanks for the interest on my problem .
This is the network that I use on the output of my amplifier. There is a zoble network on the amplifier ground plane and the output inductor is on the rear panel of the amplifier away from amplifiers output stage.
Regards
Arthur
Attachments
Hi Arthur,
This is a weird story. The inductor should prevent oscillations, but it apparently it refused to do its job. Maybe there is something wrong with that thing. What if you replaced the inductor by a resistor of 1 or 2.2 Ohms? If the oscillation stops, this might give you a clue.
Cheers, E.