AM Modulator Attenuating the Input Signal

[nathanpc]

I'm building a simple AM modulator to work on the 13.56MHz band (just a simple project to get started with RF). I found this simple design on the web, so I decided to change it a little bit to work with a 9V battery (just used the techniques to build a class A amplifier), my final design looks like this:





When I went to test it with the audio input shorted to ground so it'll act like a class A amplifier. I'm feeding in a 1Vpp (HighZ) from my function generator at 1MHz, 10MHz, and 20MHz:



You can clearly see that at 1MHz it's amplifying nicely, but as soon as I start to increase the frequency the signal gets attenuated. What could be doing this?

[KJDS]

I suspect that Mr Miller is having some influence.

Google Millar capacitance, build an LTSpice model to play with it's effect.

[Rudane]

Those frequencies are high for a basic transistor. Look up high frequency effects of BJTs.

[Fank1]

Welcome to the world of RF.
Theory says that the amplification factor, ie current gain, is a function of the transistors (ft) "among a bunch of other things"
A 3904 has an ft of 300 MHZ.
at 1 MHZ (300/1)*hfe
at 10 MHZ (300/10)*hfe
Also do your rf breadboard dead bug style in a ground plane and use coax to feed it.
Impedance matching is a big part of the equation.
When all else fails you need an big can of RF "FOO FOO DUST".

[T3sl4co1l]

Mentally add the probe and collector capacitance of about 5pF in parallel with the collector load resistor.  That gets you a 4.7k * 5pF time constant, or a cutoff frequency no higher than 6.7MHz.  Which will be lower with Miller effect, of course.

To address the capacitance, you have to use smaller load resistances.  Which will draw more current.  So like, R4 = 47, R3 = 220.  And R1-R2 will have to be lower as well, since base current will be higher.  This raises bandwidth (and shortens battery life!) proportionally.

To get around Miller effect, you can use a grounded base amplifier, or a combo that contains one, like a cascode or diff pair type circuit.  The disadvantage to these is, more transistors in series means less voltage available for your output signal.

You can also isolate the load (probe) capacitance with a buffer.  An emitter follower is excellent for this.

At very high frequencies (meaning, a low end of perhaps MHz, on up to... anything), it is convenient to replace many resistors with transformers and inductors, so that those resistors aren't loading down the signal.  If you only need a narrow frequency range, you can skimp even further and use too-small inductors for the replacement, resonating them with capacitors to preserve performance.  Now you have a tuned RF amplifier.

Tim

[nathanpc]

Thanks very much for pointing out the Miller effect and the ft, which I didn't know about.

I decided to start testing the theory that my oscilloscope probes (15pF) could be creating a low-pass filter with the collector resistor. I've started by probing the function generator output with my probes. The signal stayed the same from 1MHz to 25MHz (the top range of my function generator), but as soon as I deliberately added a 4.7k resistor in series with the probe I could clearly see the low-pass filtering.

I'll play around with the circuit in LTspice to understand the ft and Miller effect impact. When I have a circuit that is working somewhat efficiently I'll probably build it permanently in some copper clad that I bought knowing I would need it for my RF experiments.

I'll probably build a simple emitter follower in a piece of copper clad to probe this and future not-very-high frequency stuff.

Again thank very much for all the helpful answers!