Voltage amplifier for 20 Hz - 500 KC or even higher: 1 Volt AC in = 75 Volt AC out demo & schematic

Описание: Please rea type, d the complete description of this video for the best and most complete information.

The schematic of a voltage amplifier is showed. It amplifies, in general, 75 times. So 1 Volt AC input gives 75 Volt AC out.

More (perhaps relevant) info here: (29 dec. 2025)    • Видео   Though only first ideas there....So you can skip it....

The circuit now/here is not a current amplifier that can be used to drive e.g. a loudspeaker. It only amplifies Voltages in the mA (Milli-Ampère) range.

The value of the output capacitor (4.7 uF, ideally a non-polar type, but you can also use an electrolytic type, I used it here) is too small to realize a substantial current out to any source. Also the 12 K output resistors in the end-amplifier transistors will prevent that.

I used the Philips (1980’s) transistor BF 869 as one of the transistors in the end stage. It is a NPN Silicon type. Reason: its very high amplification.

I matched the BF 869 NPN (Silicon) with the MJE 350 (PNP Silicon) type in this complementary end-stage, because of its (both, NPN & PNP) high current amplification, say in the order of 100-150.

But: the BF 869 (NPN) from the 1980’s (made by Philips) can be replaced by a MJE 340 (also NPN Silicon, also with a high amplification). I am sure that this will work in this case. So this /is was a good match.

Reason: when the current amplification of such transistors in an end-stage (complimentary NPN-PNP) differ too much (say: one is 10 and the other is 100), the waveforms sent into the amplifier can not be amplifed in a pure way. Thus they must (ideally) be the same.

Though there is a quite big margin (!). Say a difference of 80-100 will do, even 50-100 in not critical amplifying applications…Strange, but true, experimental knowledge. It will work especially when the sine wave sent into such a contraption (a difference in the order of 50-100 Hfe) is not too high. When such an amplifier, made in that way, is driven with a too high amplitude, these differences in Hfe between the NPN and the PNP transistor will show up, thus could be problematic (pos. or neg.part of a sine wave amplified in a not equal way).

This issue does not (!) regard this circuit/schematic. Waveforms are properly amplified, in their positive and negative waveforms, via which they are built up. NB & please note: the exact properties of this high voltage amplifier (amplifying different wave forms between 50 Hz and 1 MC, sine & square) are showed at the end of the video, when talking about the amplification of sine waves and square waves.

Due to (say) linearity issues of this amplifier, the more or less proper (pure) amplification of square waves starts at 300 Hertz-1000 Hertz. Test, try, adapt, I am curious about your adaptations to get better results in this regard.

VERY important: the bias (= the purity of amplification, is: the "working point" of the complete 3 or 4 transistor stage) is set via the 100 K potentiometer. It has to be set to get the best waveform out on frequency “X” and the result can be checked via an oscilloscope on the output of the Voltage Amplifier, thus from the (+) of that 4.7 uF capacitor to ground, minus (-) or (0) = null.

Remarks:

4.54: I talk about the open (=not loaded) Voltage out, say the 3 x effective, thus not the nominal AC value that is printed on the transformer. Calculate this with (say) 5-10 % higher, better: measure it. It namely depends on the type of AC transformer. The open (unloaded voltage) can even go to 140 Volts DC, out of that tripler, in this case.

5.20: Like I told, it is no current amplifier, reason: explained

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