# How to design a simple current based class A amplifier

Very much along the lines of Class A single transistor amplifier with 2N3904, but with a current signal input. My circuit is a Zener noise source using the simplest of components. Traditional class A design starts with locating the Q point based on a voltage signal. I'm not sure how to proceed due to the R1/R3 relationship affecting the Zener current which should target 60uA DC. Is this architecture even possible?

My parameters are as follows. The Zener current is empirically determined for maximum noise using the ones I have.

Target gain = 10x.

Vcc = 30V.

Noisiest Zener current and Ib = 60uA DC with 10uA avalanche signal.

Bandwidth 100kHz.

I can foresee a design situation where R1 = 0 and becomes redundant. I have considered a FET based design, but was curious about a BJT design and due the fact that Horowitz & Hill do not recommend them (§ 3.08, 2nd Ed.)

• Do they say why? Certainly with a BJT design you can't count on the $\mathrm{H_{FE}}$ of the transistor over component variation or temperature. This leaves the zener diode current (and hence, if I understand the process, the noise amplitude) uncontrolled. – TimWescott Dec 23 '18 at 20:32
• @TimWescott Higher inter-electrode capacitance and Miller effect with discrete FETs. It's too high brow for me unfortunately to do anything other than paraphrase them. Sorry. – Paul Uszak Dec 23 '18 at 20:39
• @PaulUszak I just noticed this written by you a few years ago. So this is an abiding interest of yours, I gather? – jonk Dec 23 '18 at 20:42
• Wait, you're saying that AoE dis-recommends the FET design? – TimWescott Dec 23 '18 at 20:44
• The one time I did this I kept everything in the vicinity of 50$\Omega$ and used plain old RF amplifier techniques. AFAIK you just need to swamp out your amplifier's input noise with a predictable noise voltage, and then amplify it by a predictable amount. Anything else would be window-dressing. – TimWescott Dec 23 '18 at 20:47

Since Hfe varies greatly with temperature and operating current your class A design is going to have quite variable gain.

The 2N3904 has high variance of Hfe with operating current, and requires substantial current to get good gain.

Since you seem to have established the Zener current to produce the best noise output, it may be a circuit like this would be better.

simulate this circuit – Schematic created using CircuitLab

R1/R2 provide about 1mA current and set the base close to 2V. R4 has about 1.2V across it and set the operating point of the 2N3904 at about 18V.
R3/R4 set the gain to 10 for the noise signal.

• Wow, that's a neat angle on it. Though I thought my example, and the one in the link are all temperature stable. They're exactly the bypassed emitter resistor configuration recommended in all the literature, including AoE. Confused again :-( – Paul Uszak Dec 23 '18 at 23:08
• Using direct base drive is never very thermally stable, so I don't recommend it. While I used your 30V supply in the answer, I'd suggest you could easily drop this to a 15V supply with another noise source, either a Zener or a BJT BE junction. You might also find this link interesting (holdenc.altervista.org/avalanche/index.html) where the guy experimented using a 2N3904 BE junction to some success as a voltage noise source. – Jack Creasey Dec 24 '18 at 0:53
• BJT's don't work backwards for long. See electronics.stackexchange.com/q/289058/56469. The high voltage is because it's much easier to make 30V than to amplify. And I believe philosophically sounder. – Paul Uszak Dec 25 '18 at 1:25