Ft of the transistor

[Kevin Weddle]

A strong base signal could be developed at the base, bringing the base in phase with the emitter, which is the case of a common emitter or common collector.

 

[audioguru2]

The output of the transistor amplifying oscillator is the collector. If it is fed to the emitter and the transistor's base is at AC ground then the transistor is a common base amplifier and the feedback is positive and makes it oscillate.

If you feed the collector signal to the base then the feedback is negative and the transistor does not oscillate.

A phase-shift oscillator is for low frequencies and uses three RC phase-shift networks. The networks have too much loss for it to oscillate at VHF radio frequencies.

 

[AN920]

I simulated my FM transmitter and increased the value of its oscillator's feedback capacitor. It produces 11V p-p across a 75 ohms load which is 202mW RMS.

The collector current has a peak of 95mA. The transistor's collector current drops to zero on each cycle. As I suspected it operates in class-C.
The base has positive and negative 20mA peaks.
So the Hfe is 95/20= 4.75.

http://www.electronics-lab.com/forum/index.php?action=dlattach;topic=10866.0;attach=9361;image

Hi AG,

Are you sure about this? Using a non-linear simulator with vendor non-linear models for the transistor and coils (Q value 200), shows that the collector current never gets to zero, and the Vp-p on the collector is 3.4V giving only about 10mW F1 into the 75 Ohm load. AC load line analysis shows that it always operates in class A. Collector current swings between 16-41mA on the final transistor collector. This is your original circuit with fb cap 4.7pF.

With 10pF fb the output is under 30mW still operating class A. Vp-p on collector increase to about 6V.

Increasing the base resistor to 100k shows that the collector current starts to go below 0, but the power drops off to 20mW.

Have you actually measured these voltages on a scope?

 

[AN920]

AC load line and fundamental power with 100k base reistor

 

[audioguru2]

Nope, I didn't scope my FM transmitter. My scope doesn't go that high and its input capacitance might load the circuit too much.
I have seen scope photos of similar circuits producing a 30V peak at the output. Way higher than the supply voltage. The transistor becomes completely cutoff.

 

[AN920]

I did some more simulation to show the effect of increasing the feedback cap on output level and harmonics.

First I took just the output stage and performed a RF power sweep on this stage to determine the maximum output power and gain compression.

Then I measured the drive level in mW into the final stage from the oscillator. This plots show the drive with a 4.7pF cap to be about 0.15mW to the final stage.

Plotting that on the power sweep graph shows the output produces by the amplifier stage to be 11mW at the fundamental.

PO1 is the output and PO2 is the input drive

 

[AN920]

Next I increased the feedback cap to 22pF. Now the drive level was near 2.6mW (17 times higher), BUT note the rapid rise of harmonics from the osc (green circles) and we are no longer on the linear part of the power curve. The amplifier is now heavily into gain compression. Output of F1 is now 55mW and amp almost in full compression (no increase in input drive will provide higher output) as we have reached the flat part of the curve. Also note how the osc frequency shifted down as expected because of the higher capacitance.

Here is also the plot for power in transistor with 47k base resistor connected with input drive stepped 1-10mW.

 

[AN920]

Finally, I biased the amplifier stage into class-C with removing the 47k resistor and adding a RF choke to gnd on the base. I ran a power sweep again. Now we can see that if we can get the drive level up to about 8mW (which it seems we can't) from the simulation data, it is possible to get up to 150mW into the 75 Ohm antenna or load before gain compression for this class starts to come in.

The 2nd last plot shows the Vp-p on the collector as the drive level is stepped up. Distortion is clearly visible as the drive level is increased to maximum (10mW)

Last plot shows power dissipation in the transistor (Watts) for various drive levels 1 to 10mW.

 

[AN920]

So, how do we get more power? Well running the osc on 9V gives us almost 8mW drive and higher harmonics, which gives us about 130mW on the power curve. The problem is that now the frequency will drift as the 9v battery gets flat. So no easy answer apart from adding another amplifier stage.

 

[audioguru2]

My transmitter takes the oscillator's output from its emitter. Many other FM transmitters take a higher level oscillator signal from the collector or from a tap on the coil for a higher output power. I didn't try it.

 

[AN920]

Here's plots from swcad using vendor coils from their library with Q of about 100, which is about normal from winding a 100nH coil. Feedback cap is set at 10pF. Now the collector current does not go under zero. Vc is the voltage on the collector, Ic the collector current and VL the voltage over the 75 Ohm load. Ic was meaured by placing a 0.001 Ohm resistor in series with the collector terminal and using the current probe. Model for 2N3904 is still linear SPICE model and not the more accurate non-linear model used before.

Note:For some reason (better resolution?) swcad change the scale when the collector voltage and load voltage swing are plotted on the same graph. Load voltage should swing negative and positive because of the coupling cap. When it is plotted alone on a graph, it shows correctly. The p-p swing indicated is still correct.