Simple RF one transistor amplifier

Thread Starter

tonyStewart

Joined May 8, 2012
31
Do you know how to make a negative feedback amplifier from 1 or more transistors?

We know the properties of an ideal Op Amp are 0 input current, low output impedance from negative feedback using excess gain (f), from the impedance ratio, Av-= - Feedback/ Input . Although the transistor collector current source is high impedance (Z) with voltage gain and the emitter is the source reduced by the current gain for each stage.

If you want a simple linear amplifier with 1 transistor you have a lot of tradeoffs with impedance reduction or current gain and/or voltage gain. But by using negative feedback for DC and AC, you can AC couple with higher gain and lower sensitivity to the typical 4:1 ratio of max/min hFE DC current gain specs in datasheets.

With H bias you cannot get full swing output without tuning for this hFE variation. Even when you tune h bias for maximum swing, the +ve distortion from lower Vbe causes lower DC current and thus lower gain causes asymmetry and the -ve output peaks < 2V approaching saturation where hFE reduces rapidly.

This asymmetric difference in +/- peak gain to Vavg is almost equal to the Total Harmonic Distortion THD (%)
Falstad Sim. with slider variables RF amp 50 ohms load, gain = 23 dB
1666895322219.png

1. I created a linear RF amp using the above. I applied an RF input sweep with a gain of 20 and 5.4Vpp output from a 3.3V supply into 50 Ohms.

2. Although the NPN was biased to 1/4 W that makes the intrinsic base resistance much lower than 50 Ohms thus adding Re= 750m Ohms times hFE raises the input dynamic impedance for linearity somewhat.

3. The collector draws from 10uH to Vcc the current that creates high voltage gain, attenuated by the negative impedance such that Vbe variation is reduced and also THD.

4. The 4k feedback to base 50R input achieves this gain with a 50R load but needs additional DC current provided by the 10k pullup to the base.

5. Both inputs and outputs are AC coupled to minimize settling time and minimize AC voltage loss for the frequency range used.

6. Unlike an Op-Amp there is far less DC open loop gain to work with but enough to utilize the gain bandwidth of the single NPN transistor so that an internal integrator is not needed.

7. More exotic variations of this exist for higher frequencies but I thought this simple version would make it interesting and more useful than the H bias versions of Class A single transistor amps for a larger linear swing.
 

MrSalts

Joined Apr 2, 2020
2,603
If I was building a working circuit I would use a 2N5106.
The datasheets looks like New Jersey Semiconductor Products Inc distributes parts made by other manufacturers - note that their datasheet lists "manufacturer" as a line item. In this datasheet, Manufacturer is listed as "many". Although I've never dealt with that particular manufacturer, I am sure they are a high quality vendor. Unfortunately, "many" does not list the Hfe or hfe at 150MHz. The posted one-transistor amplifier design highly depends on the gain of the transistor - especially the emitter resistor. So, I'd suggest anyone building the amplifier to try 0.75 ohm but it will more likely need a 6.8 to 33 ohm resistor to get a symmetric waveform and gain my not meet the target of 5.4v p-p in the first post on this thread.
 

MrSalts

Joined Apr 2, 2020
2,603
Let's keep this on topic. I am curious what the TS used for a transistor also?
I thought he was simulating off of a design you built and posted in the original thread above.
Now I'm confused by who did what if you weren't the source of the info. But the symmetry will be dependent on the gain at the given frequency.
 

Wendy

Joined Mar 24, 2008
23,065
Due to the age of that thread, it had been deleted. But it was a good enough post I decided to keep it intact. Please stop stirring and drop it.
 

MrSalts

Joined Apr 2, 2020
2,603
Due to the age of that thread, it had been deleted. But it was a good enough post I decided to keep it intact. Please stop stirring and drop it.
As I said, twice, It has been clear from the beginning what happened about the old thread. I have simply been asking about the Hfe that was used for the design of the circuit - the Hfe used for the circuit your original thread and Hfe of @tonyStewart 's modification of your circuit.
 
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