Andy,
It is a pleasant surprise that you sat through it all!
These amplifiers need 'strong' terminations at both ends. If two amps cascade, this is usually lost. A trick is to add a 6dB attenuator between the stages. However, I havent noticed such a drastic drop of gain. There is a catch though.
Are you sure the second stage is not saturating? This is where a step attenuator is really effective tool to have. Reduce the input power by 10 dB and check?
The isolation resistors are chosen for a compromise between sufficient isolation and the current draw.
If your amplifier is drawing 20mA current, then a 220 ohms resistor will show 4 volts of drop! I usually gey away with 10 ohms unless the amp will draw more than 50mA. Usually with higher current, you have to increase the bypass capacitance and decrease the isolation resistance to keep the RF from leaking in and out of the DC line.
A more elaborate technique, often used with power stages is to use an inductor in place of the resistor. This is not a casual choice. You must not use the cheap molded chokes. They have a self resonance frequency well below the RF frequencies of our operations. This means that they are behaving more like capacitors rather than inductors! Instead, choose an inductance that has around 50 ohms reactance at the lowest operating frequency and wind it on a ferrite toroid. Remeber to bypass both ends of the inductor.
Farhan - it's still an excellent video and calculator - thank you!
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Are there any rules of thumb about selecting the isolation resistor for the power input? I've seen from 10 to 220 Ohms. Is it calculated like a current limiter, or??
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I've been experimenting with connecting multiple stages of amplifiers together.? I was surprised at the interaction between transformer coupled stages designed for 50 Ohms in and out. After tweaking a W8DIZ 2SC5551A amp for 17 dB of gain, gain of the first stage jumped about 5 dB after connecting? the second 17 dB stage. Does that sound normal?
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There's art and science in this RF stuff.? Thanks for your work, and for easing the transition to W7ZOI'S feedback amp models.
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73, Andy
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On Sat, Jun 20, 2020 at 10:20 AM, Ashhar Farhan wrote:
Gang,
I have recorded a video on how to build feedback amplifiers. It is little?long. Do take a look if you are into homebrewing. I explain what a feedback amplifier is, how to choose the component values, how to build it and how to measure all the aspects like gain, input and output impedances and IIP3.
- f
