amp with mosfet output

Here is the circuit as designed.

I printed the schematic and taped it to a board. I thought I would drive a pin at each dot in the schematic. That cherry board as to hard so I drilled a hole at each dot and screwed a lug to it. Then it was just connect the dots with the parts.

It is very stable. No motor boating or whistling. I connect my fluke meter to the output on it lowest range and read .02 millivolts. I connected my AF generator and set the output to 5 millivolts then checked the input and it was less than a millivolt on the input. I set the meter to read db amd adjusted the input to -50db the output read about -20db. The reading were at different resistances but the meter would not read the signals at the low levels used in the test. (the reading are relative to the voltage drop at each point)  I connected a speaker from a telephone hand set and it was room filling volume. I think this one is the best so far. Maybe it will be put on a metal ground plane with a front panel for the input and output jacks, switch and volume control.

Note: A 2N7002 MOSFET should work. The other transistors are general purpose Germanium AF transistors. You could use silicon transistor but would need to adjust the bias.

TRF radio from a 50's magazine

This is from a 50's magazine. It has a tuner and detector feeding an AF amp. I made a test circuit and tried a couple of my in stock transistors to see how they match the sim. It appears the AC128 is a good match. I only changed the transistors and bias resistors in the series of test.

1 mV will drive my earbud to a good level so it look good. You can use a high Z headphone connected where L5 is.

Adjusting the capacitor values could improve the low response but it is good enough for my ears.

I reversed the transistors and adjusted the bias. The shots are forward and reversed transistors. 

The circuit works both ways but the "proper" hookup has about twice the output. When I rotated the transistors I did need to change the bias. This will be my AF amp for the 50's receiver. I need to put a tuner ahead of it.

Transistor Theory and Practice by Rufus Turner

 This book was published in 1954. It gives the theory and construction techniques in an age that the devices were still being developed. A very good read for anyone desiring to understand the devices. Mr Turner made transistors from high back Germanium diodes. His articles were published in several magazines in the 50's.

Transistor Theory and Practice

Audion Oscillastor in sim

The Audion Oscillator is a very straight forward circuit which should perform well. The common problem with an audio oscillator is oscillations so they took an audio amp and added feed back to make it oscillate.

It looks like one of the early AF amps with a tank in the output where the head phone would be. R1 is the output tap and C4 is the feedback. The CB amp is non-inverting so the feedback is in phase with the input.

The green is across R1 and the blue is the tank. If this circuit was build with a plug for the coil it could be used in a multi band set.

I adjusted the sweep to get a better look at the signals. Looks like it is time for a build.

effect of reversing an alloy junction transistor in the circuit

Making an amp is fairly simple at first glance. Just put some forward bias on the transistor and apply a signal. The circuit can misbehave is several different ways. A transistor has three regions it can be biased to.
1. saturation
2. cutoff
3. active
With no bias it will be at cutoff.
With to much bias it will saturate.
When it is biased in the active region it will amplify.
It can be biased too much and then be over driven and distort too.
So the question is why a circuit will not operate when built "properly" but will work when the transistor is rotated.
I took two transistors and put them in my transistor hfe testers.

The one on the left is in the socket properly. The one on the right is reversed. They both show hfe but the one connected properly is much higher.

I reversed them both. The one on the right is correctly inserted now. So what does this tell us about the circuit in question?
hfe is the forward transfer or ß. The collector current will be Ib * ß. If the circuit is biased into saturation with the transistor inserted properly and you reverse it ß is reduced so it is no longer saturated. In order to give more help I would have to see the circuit in question but the simple answer is to reduce the bias.

Note: The transistors being tested are germanium micro alloy.

building a 2 stage PNP amp - with design data considerations

I want a low Z input and a low Z output. For max power transfer you need Z match, To reduce power loss you want Zgen << Zload. If the generator Z is zero ALL power is sent to the load. If the generator Z is infinite no power is consumed. When Zgen = Zload half the power is consumed in the generator. For this amp I want 50 ohm in and out. So what choices have to be made, what options are there?

So from the chart CB 100k - 500k Rout and CC 150K - 300K Rin. CB 30-1k Rin and CC 1K - 20K Rout.
So a CB feeding a CC would be:
30 ohm in - 100k out feeding 150K in - 1K out.
Loading the previous stage is to be avoided so I listed the low values for each parameter. I want 50 ohm in and out . The 1K out is the only problem. I found some 1300 : 8 ohm transformers for $.70 each so I use one and it all works out.

The circuit is very simple. 2 transistors, 4 resistors, 4 capacitors and 1 transformer.

100uv input will drive my earbud.

The frequency response is flat.

The as built. I took an old battery apart and nailed the connector to the breadboard. Using finish washers and screws allows changing the transistor. The MP40 works well.

With the battery plugged in it is ready to try. I have some V-Caps on order from Peebles Originals. When I get them it will be time for a build.

For the math minded a chart from TI.
EDIT:
I put a speaker on the output and it was easy listening level. I put a high Z headphone across the transformer primary and it was good volume too.

EDIT:
I put a tuner and antenna on it and tried some other transistors. It worked with all I tried but I think the GSS109 output and P416B was the winners. I tuned a station and changed parts. The signal was fading a little so my test could show different results if I tried it again. Some transistors made little difference while others seemed to give a boost.

Hybred as built

It was to sensitive and need some high frequency attenuation. I connected an antenna and could adjust the pot to a point I would hear radio signal. The pot adjusting the JFET into pinch off would detect an AM signal. I put C6 and C7 in the drain circuits to bypass the high frequencies.
My AF signal generator would drive the amp to hard with the output set to minimum even with the 20db attenuator switched in. After adding the bypass caps I can adjust the pot from min to max without it breaking into oscillations with an antenna and ground connected.

Without the filter it has high gain through the MW band.

With the filter the roll off is well below the MW band.

The AF response is good for my ears. The filter could be adjusted to modify it to suit.

I used a 2K pot for R11. It pinches the JFET off at about 50% adjustment. A 1K pot might be better, I have some 2K's in the junk box and used what was on hand.

C6 and C7 could be 1n to 100n. The larger value will give more bass boost.
EDIT: R11 is a 2K pot used to adjust the J112 bias. It will pinch off the Jfet at about 1K. You could use a lower value.