Add low Z output

 Note R10.

 

3 stage JFET amp

 Plenty of voltage gain just needs an output stage or transformer.

 It could drive a high Z phone but I want to drive an earbud.

 

Why is it motorboating?

It is motor boating but stops when the meter ground lead is connected to the chassis?

Could it be lead dressing or interstage coupling through the power supply (battery)? 

 

 

2SK2539 SPICE MODEL

 .MODEL 2SK2539 NJF(VTO=-3.1621 BETA=5.01519m BETATCE=-0.5 LAMBDA=2.40112E-2 RD=1.48485 RS=1.48485 CGS=1.05000E-11 CGD=1.40000E-11 PB=5.68050E-1 IS=9.11151E-16 XTI=3 AF=1 FC=0.5 N=1 NR=2 MFG=PHILIPS)

MOSFET Slider review

 If you build this one using a variable resistor for R2 you can use the MOSFET you have on hand and adjust the bias to suit the circuit. R8 and R9 represent a high or low Z load. The screen shots display the output with different MOSFET as R2 is adjusted from 10K - 20K.

 

9 volt version of AF amp

 When I built this circuit I had a 6 volt lantern battery that was needing using. I tried a 9 volt battery to compare results. My earbud is rated at 1ma and it puts out 1ma with 100uv signal. Looks like this one is a keeper?

 

 

Let's take a look at the Z transformation. Insert a resistor in series with the input and load. When the voltage drop across the resistor is one half the input the resistor equals the input Z.

 So Zin is 500 ohms and Zout is 64 ohms.

If you need an output transformer for your next build it may be worth a try.

 You could use a 2N3906. I use  the 2N5401 because I have a drawer full of them. 

 What would happen if we change the base resistor?