Imput impedance of common emitter amplifier at RF frequencies

The input is a diode junction (with capacitance). So Rin depends where you are on the diode's curve. Then there's the bias cct in parallel, as well as Tr's c_in and the stray capacitances.

NT

The data sheets of RF transistors usually have tables of s-parameters or Smith charts.

Running Spice might be easier for the OP

NT

The problem there is that the RF people rarely provide Spice models, and I suspect that the Spice models for most transistors are inaccurate at high frequencies. For example, they typically don't include wire bond inductance.

Some probably do, but I've noticed a lot of discrete transistor data sheets suck just as hard as any other kind of data sheet. I've never seen a Smith chart on one...

e.g. like this is all ya get for a transistor array that costs ten bucks?

this is all ya get for a uhf power transistor that costs 3 bucks:

Like the part has a min CE breakdown voltage spec of 8 volts and EB breakdown voltage spec of 5.5 volts but they give the S parameters at VCE of 5 volts but all the graphs are done at VCE of 3 volts?

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It has a big table of s-params.

That has a graph of Zin vs frequency, which is what the OP wanted. And it has DC curves, unusual for an RF part.

But data sheets are generally erratic and often inaccurate. I wish we could always get good Spice models.

RF small signal transistors are often intended to be used in a circuit where the emitter is grounded, and there's a temperature stable bias network set up to keep them biased properly with the emitter grounded, and the datasheet specs and S parameters are measured when it's set up like that.

I don't know that they know how to easily make a more general model for them that's valid under all conditions, when they're used some other kind of way.

unfortunately the S parameter table and the charts are at two different VCEs, idk why you would use this particular transistor in a 5 volt system it seems clearly designed for low voltage/low power applications

also you don't get anything on noise figure vs collector current :(

here is a good data sheet:

On the eval boards, the "temperature stable bias network" is usually a trimpot.

Right, they probably figure most engineering offices are climate controlled, primarily for the engineers they tend to like to be temperature stable, too

ing at RF frequencies ? The output impedance id the

or. All hints, suggestionas are welcome. Thanks in advance.

I find them inaccurate at low frequencies. When I use Spice it's as often a s not to model taking into account all those edge situations eg varying Vbe , beta, supply line variation & so on, and Spice is no star at that level o f detail. But it helps me clarify what's going on and I can work out the re alities then.

But as a general suggestion, the OP will learn a ton by getting Spice runni ng.

NT

if u think the BJT models are bad try the JFET models sometime...

Some jfets have a 10:1 range of Idss on the data sheet.

Is that part still available?

Most fast bipolars are gone now.

IDK if it's still in production but there sure seem to be a lot of them still out there for sale:

If the choice is between using a discrete JFET in a design and eating soap It's usually a toss-up except, if the JFET is used as a diode.

I have an old Roland drum machine that uses a JFET in the accent circuit, some 2SK...etc part from Japan that's no longer available at reasonable price, it went bad and it's never worked right with any of the supposedly compatible NTE-brand subs I've tried.

Here's a current manufacture one with all the nice data, and looks like a Gummel-Poon spice model is available, too: