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That is set by R1 and R2 (plus any other series resistor like R12 in Prasi's diagram).
#2n3055 transistor circuits dr r draw full#
The base current is therefore needed in the circuit to drive the upper output transistor at full current. This determines the base current needed for the transistor to pass 3A (this current can be checked by measuring the voltage across the emitter resistor). The current should be set to 100mA if you don't have an autoranging meter. Manel - here is the test circuit with the ammeter added. While that will depend on local climate and season such could be controlled by a thermostat with normally closed contacts - this being in series with the power supply and mounted on the same heat sink as the output transistors. I suggest that to allow some flexibility for variations in supply voltages etc output standing current should be set to keep the output transistors case temperature below 60 degrees. While one could buy a single wire-wound pot and use this for setting up a pair of channels, the mention of an DIL 8 switch made by Grayhill raises the prospect of making a ladder switch at far less cost. The standing current in this amplifier can be flexible as long as it is near enough to meet the need depending on the power supply voltage. is passed through an inductor like a wire-wound potentiometer there will be some alteration in phase the effects of which I think ought not to be recycled into the forward transfer path - however small these may seem. The bootstrapping still relies on the negative feedback doing the job of error correction at the same time and both loops originate from the output and both deal with a.c.signals. It is this which allows slightly greater voltage swings than with constant current sources and slightly more power. While bootstrapping increases the apparent collector load it relies on a form of feedback from the output having positive phase involving an element of a.c. I agree that pots always seem to lose performance after a few years, so I agree with the recommendations for a fixed resistor, and that is my preference over a CCS - for this particular circuit at least. In a production environment it might be as easy to use a medium power pot just to test the setting needed - using test pins - and solder in the nearest resistor to that value, especially as it would need to be a 1W part.
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If I were to build a JLH again I'd probably measure the device gain and use a fixed resistor as appropriate, as Nigel and Ian have mentioned. The main problem with using a pot (without padding resistors) is that it will be large it will probably be wire wound and certainly not cheap these days. This is not to criticise the idea of using a CCS - most Class A builders won't really care for a slight increase in power, I suspect, but I do think the original circuit is more elegant. Secondly, while I agree that JLH's modification using a transistor pair instead of resistors does allow the use of a low power pot, it detracts a little from his original concept in that a bootstrap circuit, though considered old fashioned now, provides a higher output voltage and can drive the upper transistor to its Vce(sat) value instead of being limited to about a volt or so below the rail (plus a Vbe drop), so if only from that point of view requires a higher power supply voltage and affords lower efficiency than his original. A possible downside might be that a protective diode should be added to prevent damage in extreme circumstances (clipping). I have not simulated this - I may be wrong, but with a high series resistance, (note this caveat) any inductance in a wire wound pot is unlikely to be a problem, but may actually be an advantage. I have a couple of comments on mjona's observations.įirst, I do not think an inductive component will have any significant impact when in the position of R2-R12 - these are supposed to be a constant current source, and if anything, an inductance will help to increase the impedance and make a better current source.