I am trying to design a simple RF amplifier for a
1MHz AM signal (for low-power transmission). My modulator (JFET-based) outputs ~
800mV peak-to-peak. My power supply is 9V, and I want near the largest possible amplification without distortion (class A operation) - my modulator outputs a reasonably-clean sine with next to 0 harmonic distortion, so I might avoid filtering.
This is what I came up with:
As I understand, voltage gain is
Rc/Re which is 6.8 here. That should mean an output of
Vout = 800mV*6.8 = 5.44V, reasonably big and there is still a big room for non-precise Q point setting. I picked those two resistor values based on what I have at hand.
I want to set the Q-point so the output is at
4-5V. That should be at
0.74-0.59 mA. To enable a more precise setting, the lower resistor (
1.7k) is a
As I understand, the BC238 should be capable to this amplification at this frequency, the datasheet has a 85MHz transition frequency at
0.5mA, so it should still have a bandwidth of
f' = f / 6.8 = 12.5 MHz.
I built up this circuit on a piece of copper board I cut lands into with miniature powertool with a diamond cutting bit. I used an old analog signal generator to provide the test signal, set it to
~800mV 1MHz (rather imprecise due to it's analog nature, sadly), and monitored the output with a scope. With a 100MHz 10x probe
on the 1x setting I got , and on the 10x I got
I am not sure what is the real output voltage ( (this is solved, see edit 3)
In any case, the resulting signal is smaller than the expected output. I understand that internal capacitances in the transistor start to attenuate the signal at higher frequencies, but this drastic effect was unexpected for me. I feel like I might have overlooked something simple.
I have already tried adding a small (
6nF-1u, the later was an electrolyt cap) cap between ground and emitter. That kind-of fixed the gain problem (both on 1x and 10x probe settings) but distorted the signal highly, the output more resembled a sawtooth than a sine.
- what is / how to find my real output voltage?
- how can I increase the maximum working frequency of this amplifier?
- is there a simple but fatal design flaw in my circuit?
I measured output voltage at lower frequencies. This generator can only go down to
.1MHz, so from there I collected 10 data points up to
1MHz. Measurements were rather inprecise (the voltage display on my scope changed rather quickly, possibly the generator also is not perfect), but should be accurate to
+- 10mV. All measurements were done on the 10x setting.
| f | mV | |----|-----| | .1 | 500 | | .2 | 500 | | .3 | 490 | | .4 | 460 | | .5 | 450 | | .6 | 410 | | .7 | 380 | | .8 | 370 | | .9 | 350 | | 1 | 310 |
(all those number were the measured voltages with a x10 probe, so the real ones should be those x10)
The downward going trend is still apparent from this dataset. At
100kHz, I measured a
4.2V signal with the 1x setting, which I still do not understand.
Measured two more data points -
f=10kHz I got
800mV input, and at
f=1kHz I got
400mV input. Those numbers are all around what I have expected.
The measurement problem (difference with x1 and x10) might be because my probe loads the circuit differently, and I have a big (
6.8k) output impedance, but I am not sure of that.
Edit3 - probing
I am pretty sure now that the measuring problem is due to the different loading on my circuit in the x1 vs x10 mode. I experimented with a
10k voltage divider and a
10V signal (
1MHz). With x10, I measure
500mV that equals
5V, which is around correct. With x1, I only get around
1.5V, so I should really only trust the x10 readings.
So the question remaining is that why is gain dropping so much in this circuit?