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Here is my circuit: enter image description here

It is intended to drive headphones with low distortion, and be driven by fairly high impedance source (~500R).

The trouble is maintaining a very high input impedance with high quiescent current, the values for a voltage divider needed to create a 'stiff' bias voltage (10 times the base current) on the base are quite low, the values in the schematic are already assuming a generous Hfe of around 200.

I think the CCS helps with this, with the bias current remaining constant with changes in emitter voltage, so only current to the actual load will affect beta So instead the divider could be sized to 10 times the peak load current which is much lower... Correct me If I'm wrong on anything.

I'm wondering what the effect on the ratio between the divider and base current (or in this case load current divided by beta) has on the output signal (distortion etc.)?

'1/10th' seems to be just a general, minimum figure and may not be ideal choice for this..

While the CCS may allow to use higher values in the divider and still have 1:10 ratio, it could also let me use a higher ratio (e.g 50:1) using lower divider values for a bias voltage.

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  • \$\begingroup\$ Have you simulated it? \$\endgroup\$ – Andy aka Oct 9 '20 at 13:06
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    \$\begingroup\$ @Andyaka I simmed with falstad, but dont think it is sophisticated to show the difference with the stiff divider vs none at all (i.e just 1 resistor from Vcc to base), (I am not sure what the difference is supposed to be though) \$\endgroup\$ – Jay Oct 9 '20 at 13:32
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    \$\begingroup\$ As an aside, consider a JFET input stage. If properly biased, it could deliver more drive with a higher input impedance. Of course, there are tradeoffs between various types. \$\endgroup\$ – rdtsc Oct 9 '20 at 13:35
  • \$\begingroup\$ If you are unsure about the usefulness of your sim tool then what use is it to you? There are free sim tools that are very pro and very reliable. \$\endgroup\$ – Andy aka Oct 9 '20 at 13:46
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    \$\begingroup\$ It is intended to drive headphones with low distortion, and be driven by fairly high impedance source (~500R). Why on earth would you do that with a two-transistor amplifier, using really the oldest transistors that you can still buy? Wrong approach... use a proper amplifier IC, and be done. \$\endgroup\$ – Marcus Müller Oct 9 '20 at 15:08

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