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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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In Reply to: I'll bite... posted by Lew on January 10, 2015 at 11:13:00:
In the standard RC coupling (between the plate of the preceding stage and the grid of the next stage) you use an electrical component (the capacitor) that has infinite DC impedance and negligible impedance at audio frequencies (w.r.t. the grid resistor, which in turn has high impedance w.r.t. to the plate resistance).
A current sink is a device that has theoretical 'infinite' DC impedance and very high dynamic impedance (in the audio band) w.r.t. the plate resistance (and the feeding resistor).
When a current sink replaces the grid resistor, a fixed current (1 mA for example) is spilled from the plate load of the preceding stage (via a feeding resistor).
Provided that the sink current is lower than 1/5 of the cathode current of the preceding stage, the plate voltage swing is not significantly affected.
When a signal is applied, the plate voltage swing is then 'transmitted' to the grid of the next stage because both the tube grid AND the current sink (that are in parallel) have 'infinite' dynamic impedance.
In order to calculate the voltage across the feeding resistor and the current sink the math is the same as with any voltage divider.
You have however to carry out two separate calculations: one for DC and a second one for AC.
Once you have set the DC grid voltage to the target value (usually about 0V) by the proper feeding resistor you have only to check that the voltage swing across the current sink does not exceed the maximum value (40V for LM334Z may be too low and hence the 125V of TL 783 appear more suitable).
LTSPICE simply speeds up the calculus.
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Follow Ups
- RE: I'll bite... - 6AS7_6SN7 14:22:18 01/10/15 (2)
- Thanks so much.. - Lew 09:03:23 01/12/15 (1)
- RE: Thanks so much.. - 6AS7_6SN7 12:21:33 01/12/15 (0)
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