The benefit of filtering a detector output

The signal on the headset with no filter.

Adding the choke cleans it up a bit but still some there

Adding the capacitor to provide an RF bypass to ground produces a clean audio.

Alternately you could bypass the head phone BUT the RF would still be in the phone leads. Better to remove it before the choke.

The capacitor is before the choke in this circuit.

I would suggest trying a bypass cap too.

Crystal detector and amp

A circuit like this will be presented on the radio board forum as a circuit someone built. The board experts will proceed to tear it apart. The following series of screen shots will address the circuit operation and the surplus or lack of parts.

R1 represents the head phones. This is Ir1. as you can see it does indeed have an audio component in the output.

This is the base voltage. The transistor is biased on and receiving the AF component. It look a lot like the previous shot. Heavy mix of AF and RF.

I added a capacitor across R1. It does seem to filter out the RF. From what I see I would say the capacitor serves a purpose.

The 'experts' say the diode is redundant and the base emitter junction will detect without it. I removed the diode to test the theory. What say yea?

I put the diode back and tried a different transistor. Looks good.

Reversing the diode doesn't seem a good idea.
Enough said for now.

Sometimes we seem to think the holy grail diode will be the lower forward voltage device we can find. Testing that theory here by using a low potential diode. More on that later.

This is the best circuit. What makes it work as it does? It uses a silicon diode, a silicon transistor and a filter capacitor.

The original circuit used a germanium diode and a germanium transistor. The key is matching the components. Look at the diode polarity. When the input goes negative it passes threw the diode and charges the .001 capacitor. The capacitor then biases the transistor and turns it on. Without the diode the capacitor would discharge on the positive half cycle. Turning the diode puts the wrong polarity charge on the capacitor and turns the transistor off. The original circuit doesn't have the filter across the headphone, it should. Some say the phone leads will provide the capacitance. While it may be true having RF in the phone leads is not good practice. Some say hand capacitance is a big issue. Perhaps it would not be if the phone leads was properly bypassed.

RF amp test results

First I should clarify the "unconditionally stable". If the circuit is built using standard RF practices it is stable but because it is so sensitive it will pick up strays if not grounded and shielded. I did not have any 220k resistors on hand so I used 2 100k. I left all the leads full length and the leads made antennas causing cross feed. So I reworked the bias with the leads clipped short.
I connected the output to my AF amp and powered it up. I could hear some static. Touching the input I hear my local station.
I will do more test later but it appears to work as designed.
EDIT: I used 200nfd caps in my build.

Adding a second stage to the broadband amp.

I left the first stage as built in the last post and added a second stage designed around a 1 ma current.

I changed both stages to low current and checked the total current.

The frequency response of the 2 stage circuit.

  10 uv on the input

100 uv on the input.

1 mv on the input.
As you can see the second stage is not loading the first. Time for a build?

Low current - high gain - high bandwidth

picking number from thr air vs designing

I will try to explain how I design the amp with little math and datasheet supplied data. First I pick a transistor. For this example I will use the KSP10.

The three things to look at are voltage, ß or hfe, and frequency. I will be making a battery operated amp so voltage is good. My frequency is 2 MHz so good here too. ß is 60. So how to use this info?

This is the circuit I will use. Now for more assumptions. I want Zin and Zout to be 50 Ohms. I want Vce = 2 volts. This is not the best of amps but will help with the understanding of my methods.

I don't like high current drain circuits. With a 3 volt supply and 50 Ohm load resistor I will be dropping 1 volt across R1. The current required is 20 ma. Knowing ß is 60 allows me to size the feed back resistor so R2 is 3250 Ohms. I run the sim and find Ir1 = 20ma. So my Q current is good. It is fairly simple and the circuit is 100% stable. I can hear someone ask how? The base voltage depends on the collector voltage which depends on the base voltage. Going in circles like that creates negative feedback. If you drive the collector to saturation you cut off the base bias which pulls the collector out of saturation. Neat huh?

Hear I check the collector voltage. I was looking for 2 volts. Looks good.

As I have said many times before get the DC set and then add AC components. This is the frequency response and gain I get.

Here are the signals on the transistor. The red and blue are the transistor base and collector. As you can see they are out of phase and this makes for a stable circuit. The gain is rather low so what can be done about that?

As I said before the AC impedance and DC resistance are different. Here I added a coil to 'boost' the AC impedance to increase the circuit gain.

 The coil value would depend on the desired frequency. I changed the coil value to see the effect.

I changed the output to see the effect.

This a a low Z circuit so draws high current and does not have a very high gain. Next time how to reduce current and get higher gain.

The AF amp build

In the final build I added R11 to 'soften' the input. It has something to do with capacitors charging and discharging. The circuit works better with the 200 Ohm resistor. The prior version worked fine as long as the feed had no DC in it. Feeding though the capacitor is the better option.