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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 31st Jul 2018, 6:19 am   #21
Radio Wrangler
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Default Re: Amplifier circuit problem.

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1N4007's, chuck em David? Have they no use whatsoever?
They are very slow diodes, actually built as PIN diodes to get the voltage rating. Used as rectifiers in 50Hz supplies they are slow enough to cause reduced efficiency but the real killer is that they turn off late and with a snap. The snap creates quite a fast pulse which creates radio interference by radiating from mains leads. To combat this, a lot of gear had to have extra-large suppression capacitors across the mains just to pass EMC tests..... yup, Rifas!

Used in a switching supply, the things just overheat and barely rectify at the frequencies involved.

Ironically, they do have uses, just not as unidirectional conductors. They can be used as variable capacitance diodes. They don't give the wide tuning range of hyper-abrupt varactors, but they are cheap. People have also used them successfully as PIN diode RF switches on the output of transmitters.

They do seem to be ubiquitous and the first diode most people think of. It's a parallel of people spraying WD40 all over everything. For rectifiers, use newer 'ultra-fast' types. They switch cleanly, with less interference, and better efficiency. For handling signals, 1N914, 1N4148 etc do the job. For clipping/clamping, you need beefier signal diodes and to look at the voltages and currents on a case by case basis. 1N400x are too sleepy to do any good, and the higher voltage ones are worse than the lower voltage ones.

The interference thing is quite real. I've measured up to 30dB reduction in emissions just from swapping rectifiers in some equipment.

In general: best avoided.

David
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Old 31st Jul 2018, 9:29 am   #22
Argus25
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Default Re: Amplifier circuit problem.

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Originally Posted by Diabolical Artificer View Post
Could you post the circuit please?
Hi Andy,

Circuit attached. In this case T1 was used as a comparator to determine where it clipped.

(In the application the base current of T1 was used as part of a soft peak clipper before an AM modulator, so the actual deliberate peak clipping was visible on the LED).

You can get rid of T1 and probably just drive T2 via a coupling capacitor, or an op amp set up like a comparator for some slice level. (diodes 1N4148)

The main thing about it is that T2 sends a good pulse of current on the peaks via the diode and 100R resistor to rapidly charge the 1uF capacitor, then, between peaks, it discharges more slowly into the base circuit of T3 which keeps the LED on for a good amount of time after each peak.
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Old 31st Jul 2018, 10:21 am   #23
Herald1360
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Default Re: Amplifier circuit problem.

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Originally Posted by Diabolical Artificer View Post
"Ensure that whatever happens, they can't get more than a few 10s of mA through them." For a full HT fault that'd mean 20k R's in series with the diodes I presume. Might be a problem, room on PCB is very tight.
Not in series with the diodes, in series with the signal input to the diode centre point.

In your case, there is no need for additional protection resistors. the source impedance is taken care of by the anode load resistor and in any case there's a dc block from the 10n.
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