AF Amp with gain control

I put a coil. V-cap and diode feeding the amp and found the volume to high. I added a gain control.R15 is a 2K rheostat. It could be placed in  Q4 collector circuit. This would limit  the low end.

With the rheostat in Q4 circuit it swings the output from 2 mv to 18mv.

AF Amp circuit operation

Q4 is a preamp.
Q3 is a phase splitter.
Q1 and Q2 are a push-push amplifier.
* connecting a center tapped transformer in the output circuit would produce a push-pull amplifier. This is how I was taught 50 years ago. Today the terminology may have changed?
Anyway the key is Q1 and Q2 bases are being driven out of phase. Q1 and Q2 operating class B (conducting 50% of the time) feed the load a full cycle with one transistor biased off while the other conducts. (or AB with one being throttled down while the other is driven harder.)

When Q2 is driven into conduction current flows DOWN thru C1 and R3 to ground. Charging C1.
When Q1 is driven into conduction current flows from C1 DOWN thru Q1 to ground and UP from ground thru R3. Discharging C1.

Here we see the phase relationship between the signals.

The one thing to watch here is not to use to small a capacitor. A large capacitor will receive a large charge and provide the power on the next half cycle.

I set R1 and R2 to 4 Ohms with C1 at 1000ufd. 5ma battery drain and 180mv output with the same input as before.
It is a good earbud driver as I made mine but you can adjust the bias and add more power as desired.
Good luck!

another attempt at a low power low Z amp

The transistor output Z is to high to feed a speaker or headphone. One way to overcome this is to raise the collector current. A higher current will reduce the output Z but it cost power. I want a battery powered set that will not drain the battery to quickly. A BF2040 stage will feed a 50 Ohm load with 10ma current drain. I want to try and do a little better than that. We could use an output transformer but I want to use parts available off the shelf. While winding a transformer is not extremely difficult is not beginner level. I need 1 mv to drive my earbud. This circuit works faily well.

Driving it with 25uv gives me the 1mv out.

100uv gives  over 4mv. So far it looks good but what about the power drain?

1.5ma current drain at 9 volts is not bad. This could be my circuit for my new radio build.

It drives my earbud with the output set to min on my AF generator. I connected a speaker to it and could hear the tone across the room with the generator on about 5%.

My favorite hearing aid amp is very good but it requires transformers.

This one works well. I have some transformer in the parts been that work in it. If you don't have any transformers and don't care to wind any the circuit above is a good alternative.

100 mhz amp I -O and script

I set  the generator to 100 uv at 75Ohms. Half the signal is being dropped in the generator.

 I moved the generator and battery on screen to show the whole picture.

You can cut and paste the spice script below.

************************************
Version 4
SHEET 1 880 880
WIRE 128 80 128 64
WIRE 400 112 400 64
WIRE 32 144 -32 144
WIRE 336 160 128 160
WIRE -112 176 -128 176
WIRE -32 176 -32 144
WIRE -32 176 -48 176
WIRE 32 176 32 144
WIRE 128 176 128 160
WIRE 592 192 528 192
WIRE 624 192 592 192
WIRE 400 224 400 208
WIRE 528 224 528 192
WIRE 624 224 624 192
WIRE 80 240 32 240
WIRE 160 240 80 240
WIRE 32 256 32 240
WIRE 80 256 80 240
WIRE 160 272 160 240
WIRE 528 336 528 304
WIRE 576 336 528 336
WIRE 624 336 624 304
WIRE 624 336 576 336
WIRE 32 352 32 336
WIRE 400 352 400 304
WIRE 400 352 368 352
WIRE 416 352 400 352
WIRE 576 352 576 336
WIRE -32 384 -32 256
WIRE 80 384 80 336
WIRE 80 384 -32 384
WIRE 160 384 160 336
WIRE 160 384 80 384
WIRE 368 384 368 352
WIRE 416 384 416 352
WIRE 80 400 80 384
WIRE 368 512 368 448
WIRE 384 512 368 512
WIRE 416 512 416 464
WIRE 416 512 384 512
WIRE -656 528 -656 512
WIRE 384 560 384 512
WIRE -656 624 -656 608
WIRE -560 656 -656 656
WIRE -656 672 -656 656
WIRE -656 768 -656 752
FLAG 80 400 0
FLAG -128 176 RF
FLAG 32 352 +Vcc
FLAG 128 64 +Vcc
FLAG -656 768 0
FLAG -656 624 0
FLAG -656 512 +Vcc
FLAG 400 64 +Vcc
FLAG 384 560 0
FLAG 576 352 0
FLAG 592 192 OUT
FLAG -560 656 RF
SYMBOL npn 128 240 M270
SYMATTR InstName Q1
SYMATTR Value BFR193
SYMBOL res 64 240 R0
SYMATTR InstName R1
SYMATTR Value 820
SYMBOL cap 144 272 R0
SYMATTR InstName C1
SYMATTR Value 10n
SYMBOL res -48 160 R0
SYMATTR InstName R2
SYMATTR Value 7k2
SYMBOL cap -48 160 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL res 16 240 R0
SYMATTR InstName R3
SYMATTR Value 1k4
SYMBOL res 112 64 R0
SYMATTR InstName R4
SYMATTR Value 7k2
SYMBOL voltage -656 656 R0
WINDOW 123 24 124 Left 2
WINDOW 39 24 152 Left 2
SYMATTR Value2 AC 1
SYMATTR SpiceLine Rser=75
SYMATTR InstName V1
SYMATTR Value SINE(0 100u 100meg)
SYMBOL Misc\\battery -656 512 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 9
SYMBOL npn 336 112 R0
SYMATTR InstName Q2
SYMATTR Value BFR193
SYMBOL cap 352 384 R0
SYMATTR InstName C3
SYMATTR Value 10n
SYMBOL res 400 368 R0
SYMATTR InstName R5
SYMATTR Value 17k
SYMBOL ind2 384 208 R0
SYMATTR InstName L1
SYMATTR Value 1µ
SYMATTR Type ind
SYMBOL ind2 544 320 R180
WINDOW 0 36 80 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName L2
SYMATTR Value 10µ
SYMATTR Type ind
SYMBOL res 608 208 R0
SYMATTR InstName R6
SYMATTR Value 1k
TEXT -688 832 Left 2 !.tran 0 5u 4.5u
TEXT -680 864 Left 2 !K1 L1 L2 1
***************************************
The code is the text between the ******.
The file extension is .ASC
Blogger will not let me post .ASC files?

100 MHz amp - comparing sim to calculated

This is the circuit

Ic(Q2)

Vbe

I bias

V emitter

Q1 collector current

Input and output

Time base adjusted to show waveform

The math and the model follow each other fairly well. The math was using ß = 93. I think my model uses ß = 125. I expected a little variation because of this.

I assumed a 1k load and used the 4T:14T FT37-61 transformer. The transformer could be adjusted for the desired Zout.

complete receiver - less tuner

Edit: I will have to do some testing but the build doesn't function. The AF is great but the RF amp failed. I may have a bad hookup. It will require some testing.I injected a signal into the detector and it looks good.

EDIT 1: I had R5 connected to +Vcc. Connecting it to -Vcc should fix the problem. I'll try that next shop time.

EDIT 2: I moved R5 from +Vcc to -Vcc and it was like flipping a switch. Looking good now it just needs a tuner. That will be my project for next week.

The frontend - RF amp and detector

I used the P416 Germanium transistor for the Rf amp.

Looks good now to add a detector.

The green is the generator output.
The blue is the detector output.

It has 30 uv pk-pk input and 150 uv pk-pk output.
Here is the circuit.

Time for a build.