small amp

[CBS240]

I was just thinking of an amp, not too big, but loud enough to be heard. So I came up with a relatively simple circuit to output about 25 watts into 4 Ohms, full range. In reality with full range it may not actually do 25 watts. But anyway some details…

Vas current is 6mA. Diff current is about 10mA each side. 2N6038 is a darlington transistor. The zener is to protect outputs in case the Vbe multiplier is turned completely off. The 1uF is to prevent cross conduction.

Now I have not tested or built this circuit, just thinking of the parts I happen to have sitting in a drawer. Frankly, I just made it up. But do any of you see any potential problems I may run into?

[Workhorse]

Hi CBS,

1..Replace the VAS diode at emitter with resistor of adequate value..
2..use emmiter degeneration for current mirrors at differentials.
3..Driver transistor emitter resistor 2.2k is too high and use around 220E for driver resistor .. and directly connect the drivers collector to rails ..omit 10E resistors..
4..1k preset at emitters of differential transistor is high value replace with 220e preset...
5.. replace the feedback resistor 47k to somewhat more resonable 22k to balance the differential impedance for Zero DC gain...
6..VBE multiplier resistor values are not accurate....recalculate these values so that their value fall under 1Kohm region not 100K domain..

hope this helps..
K a n w a r

[CBS240]

Originally posted by Workhorse
Hi CBS,

2..use emmiter degeneration for current mirrors at differentials.
4..1k preset at emitters of differential transistor is high value replace with 220e preset...


hope this helps..
K a n w a r

Hi Kanwar,

Thanks for the reply. I have made the changes as to your suggestions. The 1k preset on emitters of diff. was certainly too high, I wasn't thinking of how much voltage drop would be there and I would have run out of volts! I do have a question about the emitter degeneration for the current mirrors on differential. Is the 50 Ohm resistors placed correctly as to were you are refering here? I chose that value to match the voltage drop across the 100 Ohm on emitter of VAS. (0.6V)

As I said though I have yet to actually build this circuit to see any results, just theorizing.

edit>>I just realized what you meant by matching the impeadences of the two differential transistors. I have changed the 10K to 22K, and the 220Ohms to 470Ohms in order to get the right gain. I plan to drive the input with like 0.3VAC. I guess to have more closed loop gain may produce extra noise? Of course it is late here, and I may have miscalculated.

[Workhorse]

I think you are now on right track go ahead and see what happens next......
K a n w a r

[CBS240]

Thanks much Kanwar,

I will be working out of state for another couple of weeks, but when I return home I will begin fabricating and see, or rather hear what happens.

[jaycee]

You may want to add a 1K resistor in the base of the transistor that is the VAS current sink. If the amp is driven into clipping it can happen that the VAS saturates, and this sucks all of the current through the current sink's base, which then causes the current to drop in the LTP. A 1K resistor limits this effect.

Also make your rail decoupling capacitors larger, say 100nF-10uF, as small values can cause oscillation.

10mA of LTP current is rather high! 3mA would be fine here. I would use individual 220 ohm resistors to emitter degenerate the LTP, and use the preset to adjust for dc balance.

[Workhorse]

Thanks Jaycee for further resolving the CBS amp's glitches which were left out by me ....

CBS...apply the 1K resistor on the base of CCS and also use larger caps for decoupling 100MFD + 100nF

Reduce the LTP current to 4mA...

K a n w a r

[CBS240]

Does it make a difference in speed of error correction by the LTP that the current gain bandwidth product for these devices is higer at 10mA than it is at 4mA? At 10mA I am still well below the max power rating of the device, like 60% or so.

[ilimzn]

Most of the speed gained in the LTP will actually be losed when compensation is applied (and it will). In principle, there is nothing wrong with optimizing the LTP current for GBW, except that you get rather high currents, so you also get a higher DC bias current. The latter may or may not be a problem.
Other than that, you get lower Re which may also lower noise. However, with higher bias current, you need smaller resistors in the emitters, otherwise what you gain in gm, you lose for the degeneration effect. Commonly a degeneration resistor of at least the same resistance as Re is used (Re = Vt/Ie = Vt/2It, Vt=25mV, It = Itail of the LTP). At some point, increasing the emitter resistors does not further enhance linearity in a meaningful way, but does reduce open loop gain. If high open loop gain is a problem when you apply degeneration of the input pair, perhaps a better idea would be to reduce it by limiting the VAS gain, by giving it a resistor to work into (and not just the output stage).