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I am trying to build a simple CE amplifier for an AM signal as part of a home-brewed transistor radio. I would like to have a working simulation before I get a breadboard. My simulation works in small time-steps but not large time-steps and I have no idea why. Falstad Simulation

Before you make fun of my mistakes, I am a hobbyist and BJT biasing is very confusing to me. I chose fixed bias topology because it seemed the simplest.

What am I doing wrong with this simulation? Any advice/rules of thumb I can incorporate? Small timestep

Large timestep

EDIT:
I have built the first BJT stage on a breadboard following the exact schematic as shown. My expectation is that the first BJT stage should giving me a larger amplitude RF signal whose frequency is equal to what my antenna would pick up (AM radio stations in the range of 500khz-1.6Mhz).

I am probing ground to the output capacitor with a multimeter and get no voltage, AC or DC.

All I have on the breadboard is:

  • VCC/GND wires from 5V power supply
  • The first bjt stage (same as schematic)
  • A 3' length of wire->10nf input cap->base of BJT (2n2222)
  • Another 3' length of wire that is plugged directly into ground to make a dipole antenna

I probed between emitter pin of BJT and ground, 5V DC. Base pin of BJT to ground: 2.5V DC. Collector pin of BJT to ground: 2V dc.

Collector output cap to ground: 0V AC or DC.

Antenna side of input cap to ground: 0V AC or DC.

However, I am using an analog multimeter and do not think it would pick up a few mV of antenna signal.

Is my transistor biased wrong? Should base to ground voltage be closer to 1V? Are my input/output cap values the wrong magnitude for 500khz to 1.6Mhz? Did I take the simulation out of context?

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  • \$\begingroup\$ You only have one base (biasing) resistor to the supply. Using two resistors, one to supply, one to ground, will give you much more flexibility and control of the DC voltage at the base which then gives more control over the DC voltage across the emitter resistor and therefore also the collector current. \$\endgroup\$
    – Bimpelrekkie
    Commented Dec 4, 2021 at 18:00
  • \$\begingroup\$ What am I doing wrong with this simulation? - what do you perceive to be wrong? If the real question is why foulstad is not doing as expected then ask that so you are clear and be clear about what you expect it to do and what it is actually doing. fixed bias - do you in fact mean base? I am trying to build a simple CE amplifier for an AM signal - if it's an amplifier what is the BJT with collector and emitter shorted meant to bring to the party? \$\endgroup\$
    – Andy aka
    Commented Dec 4, 2021 at 18:18
  • \$\begingroup\$ Define your expectations first as Design Specs. \$\endgroup\$
    – D.A.S.
    Commented Dec 4, 2021 at 19:14
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    \$\begingroup\$ @YousifAlniemi I'm very very suspicious of any attempts to use a BJT in CE arrangement as a wideband preamplifier stage at MHz rates with an antenna and tunable tank. It's doable. But you really have to know your stuff and be able to use construction techniques that are appropriate. Personally (and I've not explored this much) I'd be tempted to look at using a JFET here. Not a BJT. The JFET is probably a better impedance match and likely lower noise at the likely quiescent current setting. In any case, expect some early failures here. \$\endgroup\$
    – jonk
    Commented Dec 5, 2021 at 6:42
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    \$\begingroup\$ @YousifAlniemi My own preference would be that you start with an air capacitor like this one, with knob and use a fixed antenna coil. That's what the antenna attaches to. Then I'd carefully design the 1st stage for this purpose. By this I don't mean just slapping parts together. I'd think about it. I'd probably use a JFET and a BJT for that 1st stage. Careful wiring is also important here. But once that's done, the rest is lots easier. \$\endgroup\$
    – jonk
    Commented Dec 5, 2021 at 6:56

1 Answer 1

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My simulation works in small time-steps but not large time-steps and I have no idea why.

The transient simulation must run with small time-steps so that the AM carrier frequency has many time samples. If the AM signal source has a 1 MHz carrier frequency, then time samples should be separated by no more than 0.5 microseconds, otherwise you violate sampling criteria.

I would use finer time steps, something like 50 nanoseconds for an AM carrier frequency of 1 MHz.

The AM diode detector after the amplifier re-generates the AM modulation on the carrier. To see this waveform, the simulation must run for a long time....
For example, if the AM modulation is 1 kHz, the simulation must run for at least 1 millisecond to see only one cycle. If your modulation is 100 Hz, you need to run for at least 10 ms.

On the one hand, you need fine time steps to properly sample a high-frequency carrier - on the other hand, you need many of those consecutive samples to see the result of the AM detector. The simulation run takes a long time to complete when the carrier frequency -to- modulation frequency ratio is large - there's no way around it.

For complex waveforms having many frequency components, a simulator's guess at appropriate time-steps may err on the too-long side. I don't know how Falstad sets time-step default. Oscilloscopes are similarly poor at guessing time-step size.
The general rule is to choose a time-step shorter than half the period of the shortest wavelength that you expect to encounter in your circuit. An accurate simulation of an oscillator would want a much shorter time-step.

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  • \$\begingroup\$ With a browser based FFT in realtime and only 2k (?) samples. Falstad Sim will work with 10k modulation to see the sidebands easier @ 1MHz and sample = 10ns and 10us/div. But front end needs more sensitivity than 10mV Offer him 100 uV \$\endgroup\$
    – D.A.S.
    Commented Dec 4, 2021 at 20:13
  • \$\begingroup\$ @TonyStewartEE75 OP's circuit could be improved, but there are no fundamental flaws. One thing at a time - the Question was about simulation. You're right, the simulation can go a lot faster with 10k modulation. Can you imagine simulating with carrier of 2 GHz? Ugh. \$\endgroup\$
    – glen_geek
    Commented Dec 4, 2021 at 20:37
  • \$\begingroup\$ Simulation is just as easy , limited only by ratio of sweep / sample which using thumbwheel tinyurl.com/yxllmxz8 but you have to add your own imperfections \$\endgroup\$
    – D.A.S.
    Commented Dec 4, 2021 at 21:39

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