Hybred Amp MOSFET input JFET output

This project is to use MOSFET and JFET amps. It works so well in sim I will have to assemble one. First the circuit.

R11 is the volume control.

I'm swinging R11 from 1Ohm to 1KOhm. With 100uv in it will rattle an earbud.

The frequency response and gain thru the AF range is good.
Time for a build.
EDIT:
You could use 2N7002 and any general purpose JFET. The 2N7002 should be a direct sub for the MOSFET. Might need a larger source resistor with a different JFET. If R11 is 5K or 10K it should work with any JFET.
EDIT2:
I used a 2KOhm pot for my build.

Looking inside the transistor

Let's look at the load vs output.

 10 ohm.

20 ohm.

30 ohm.

40 ohm.

 10 ohm

20 ohm

30 ohm

40 ohm

Now a look inside the transistor.

Here we see the emitter current and the voltage drop from emitter to collector. R=E/I is the yellow trace. The transistor Z is about 4K. It is a dynamic value which changes with bias.

Screen shots of the low Z amp

I stepped R to see the effect the load value had on power output.

With a higher load Z the output is nW.

The 50 ohm load receives uW's of power. The amp supplies higher output to a lower Z. The 470 ohm resistor is for biasing. The AC impedance is not equal to 470 ohms or I would get max power transfer at 470 ohm. What is the AC Z???

Low Z power amp feeds an ear bud

I made an amp with a transformer on the output which works well but I thought I would build one without the transformer too. The problem is the common emitter low Z amp requires more current than I care to draw from my battery. The best choice would be a common collector circuit. It would look something like this:

When a circuit like this was asked about on the radio board the board experts declared it was a power hungry waste of time and money. To test the theory I will display the circuit current draw and power gains. First the instructor said,"set the DC circuit values and then add the AC components."

I established a current drain of about 1.25ma. I could turn the generator off and see the DC but it's good enough.

In this shot the green trace is the emitter current (the 820 ohm resistor) and the red is the ear bud. Without C2 the voltage drop across the emitter resistor would cancel the input and we would see little gain.

The circuit is producing about 9nw which would drive my earbud to a good volume.

I set the generator to 300hz which is my low frequency audio.

The green is the generator and the red is the earbud current. You can see we have no voltage gain but quite high current gain. Since power is I squared * R, a current gain of 2 would equal a power gain of 4, a current gain of 3 would equal a power gain of 9, etc. This should clearly show the DC and AC should be addressed separately and the circuit can function as a low Z amp without drawing the battery quickly.
The cascode amp using this stage as its output and a voltage amp as its input would be the next step in the progression. maybe next time?

a pre amp build and test

If you have followed my blog you will realize I like circuits that are easy on the battery. So I may sacrifice a little performance to conserve battery. With this in mind look at the sims with different load resistors. While low Z circuits require higher currents we can sometimes use lower current and still get reasonable gain.

 This is the circuit as designed. 8mv output but draws 7ma.

with a 2.2k load we get 6mv with 3.4 ma battery load.

with 3k load we get 5.4mv out with 2.5ma drain on the battery.

With a 10k load we get 2mv at less than 1 ma drain. If 7ma drain is good with you the first circuit will be good for you. I like the 3k at 2.5ma draw. I have some 3k in reach so that is what I used. Let's look at the circuit.

The yellow probe is on the input and the green probe is on the output. Both traces are on the same range.

I increased the signal so the yellow trace would display a curve. I had to increase the range for the green trace. It is giving a good boost to the signal. The test was at 4Mhz. I sweep the signal to about 10Mhz and down to 100Khz. As would be expected the signal was stronger at the lower frequencies. It is impressive for so simple a circuit. 1 transistor, 2 capacitors and 3 resistors. Using surface mount components it can be made very small. I have some 1/2" tubing to try and mount an amp in. It will be easy enough to mount on a small piece of board or glass. A microscope slide could work well. Mounting a connector will be the challenge. The tubing is the same size as a BNC connector. It could be swagged to make it work. An altoid box may be better. with a divider it could hold two amps.

amp in a pipe - self biasing examined

This is the basic idea. I have made a couple on a breadboard and they perform well. I think I could use 2 for preamps on my scope and make it more useful. The low range is in milli volts. With the amp I could read micro volts. That's the plan.

node 3 and 4 are either side of R4 and node 6 is the output. When I measure node 4 and divide by the current through R4 it give me the input Z. It is a dynamic reading so we have to take the average. 150 ohms ? Basically the base emitter junction and Xc1.

I have some surface mount 2N3904 for this one. The arrows point to nodes 3, 4 and 6. I'll be taking reading from these nodes to determine the impedance and gain.

It look good.

I measured node 4 and divided by I(R4) to get the input Z.

I cleaned up the chatter from the previous shot.

The larger capacitor charges and holds the voltage to make it appear that we have a negative resistance. It is the phase shift through the capacitor.

In this shot I moved the reference to one side of R4 and measured across it. The floating ground allowed noise to display but the 50 ohm resistor is showing.

Here I leave the reference at R4 and measure to ground to get the input Z. When I did the same measurement with a 1000u cap the reading was negative. The 100u cap show a positive reading. The RC time constant is determining the reaction to frequency.

Back to the original circuit.

Here we see the 50 ohm generator Z

If I had Zgen = Zin I would read 50uv with the generator set at 100uv. How would I adjust that? Does it need adjusting?

100uv in and 7000uv out. The Zin could be adjusted by changing Ic. This could be done by adjusting the base emitter resistor or the load resistor. Changing either one would vary Vc which in turn would change Vb and cause a shift in Ic. If you plot a load line you would see the circuit gain changes with slope. So making either resistor adjustable would allow the gain and Z to be varied.

Zin is low so this would be a good circuit for a 50 ohm system. It would load a high Z circuit. Why use 50 ohm? A good question that could use a good answer. Maybe a topic for another time.

transistor action examined in spice sim

It seems the transistor and the vacuum tube share a lot of characteristics. The vacuum tube current is limited by the amount of electrons the filament or cathode can emit. Applying more plate voltage can not cause the heater or cathode to be able to emit more electrons. At very low plate voltage the space charge building around the cathode can make it harder for the emission to occur. Once the plate voltage is high enough to draw them away the current limit will be the cathode ability. It is much the same with the transistor. The emitter injects carriers into the base. (the carriers could be electrons or holes for this discussion we will assume electrons.) The collector draws the carriers from the base. The crystal structure will only support a finite number of carriers. If the collector fails to remove them the base will become saturated or loaded with them. In transitor datasheets we find the ß listed or hfe. This is the relative ability of the collector to draw the carriers away in relation to the number drawn to the base. Let's look at some pictures and see if they make this clearer.

 I applied 1 volt to the base through a 10K resistor.  This sets the base current used in the test. I am stepping the collector from 3 to 12 volts in 1 volt steps. As you can see the collector current is very close to steady state.( we'll look at the reason for the small change later.)

I expanded the scale to show the variation.The collector resistor is increased to 10K. The current is varying. Why?

The voltage Vce is at saturation. In other word the 10K resistor is limiting the collector current and the base is filling up with carriers that cannot be drawn away. To put it another way. The voltage drop across the 10K resistor would have to exceed the supply voltage for more current to flow. The transistor resistance and the 10K resistor form a voltage divider. The semiconductor resistance varies with the current flowing in the circuit.

With a 1K resistor for the load the transistor can supply the current and the voltage drop across it is close to the applied. Again the emitter inject a set number of carriers with the applied bias and the collector draws them away. The drop across the 1K resistor remains the same so the collector voltage increases as the supply increases.

With a 1 ohm resistor we see basically the same step with supply change. As I said before the collector potential draws away carriers until the out flow equals the emitter injection and then the circuit is at equilibrium. Vce is equal Vcc - Vr3.

 This is what happens when we change the current through the base. Stepping E from 1 to 10 in 1 volt steps. Look closely and you can see the family of lines. E steps to a point and V1 steps through its values and E steps to the next point. Ic varies slightly as V1 swing threw its range.

Here we have Vcc = 100 volts as we swing the base bias. The higher Vcc will allow higher currents but the transistor can still be cutoff or biased on.

Here we see the relation between the base voltage and the collector current with Vcc = 100 volts.

Cutting the Vcc in half doesn't have that much effect on Ic.

Cutting Vcc back to 12 volts still give 2.45ma. Notice the base emitter voltage is swing slightly. The transistor has a resistance which varies with applied potential. With higher voltage drawing the carriers away at a faster rate the opposition to more being injected is less and as higher currents flow the voltage drop increases. So the final change is determined by the material resistance and the injection opposition. Doping heavy increases the ability to inject and reduces the resistance but there is more to it than that.

The problem with computer volt meters can be resolution. While we benefit from the wiggly lines we can also see more than a simple volt meter will display. Here I set the range to show the base voltage swing with a Vcc from 10 to 100 volts.
The change is still there but as I said before it is steady for practical purposes. Rbe in series with the 10K resistor form a voltage divider that drops the Vbe. Generally the 10K is large enough that changes in Rbe can be ignored. That effect is showing up in my sims. I guess that would be the topic another day.

in summary.
TRANSfer of reSISTance or transistor as we say.
Low resistance emitter to high resistance collector circuit.
inject carrier into the emitter and let the collector resistance fall. We could use Fermi level, barriers etc. but the sims tell the story quite well???