The Mystery Behind a 20dB Amp at 20MHz

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SUMMARY

The discussion centers on a 20 dB amplifier operating at 20 MHz, specifically a small subsection of the HP 8558B 1.5 GHz spectrum analyzer. The amplifier functions as a unity gain amplifier when the IFG2 control line is open and provides 20 dB amplification when grounded. Key components include a pin diode with an 83-ohm impedance and a temperature compensated regulator supplying +10 volts. The purpose of inductor L7 (22uH) is to isolate the emitter of transistor Q4 from the collector of Q3 at 20 MHz, while resistor R32 manages negative feedback to stabilize the DC operating point and prevent oscillation.

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  • Understanding of RF amplifier design principles
  • Familiarity with transistor operation, specifically bipolar junction transistors (BJTs)
  • Knowledge of feedback mechanisms in electronic circuits
  • Experience with schematic analysis and interpretation
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  • Learn about the role of feedback in amplifier stability and performance
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  • Study the HP 8558B spectrum analyzer's architecture and its components
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Electronics engineers, RF circuit designers, and students studying amplifier design and analysis will benefit from this discussion.

waht
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Here I have an elusive schematic of a 20 dB amp at around 20 MHz.

It works as a unity gain amp when the control line IFG2 is open. When grounded it amplifies by 20 dB. The diode is a pin diode and supposedly has 83 ohms when the current is flowing through it.

The +10 volts comes from a temperature compensated regulator, +15 volts from a normal regulator.

This is what's intriguing, why is the base voltage of Q4 actually more than the supply voltage. It's 10.3 V

And secondly, what is the purpose of L7 = 22uH, does it defeat the purpose of R32.

I'd appreciate any insights.
 
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Have you actually measured this voltage at the base or are you assuming the schematic is correct? The purpose of L7 is to provide a certain amount of isolation at 20 Mhz between the emitter of Q4 and collector of Q3 while coupling them directly for DC. R32 determines how much negative feed back occurrs at 20 Mhz. The negative feedback here prevents oscillation and stabilizes the DC operating point. What is this for? Is this a pre-amp?
 
This is just a small subsection of an 21.4 MHz IF amp of an hp 8558b 1.5 GHz spectrum analyzer. Just looking over the schematics, it's very interesting, and educational.

Some things I find are very weird, like this schematics I showed you. If you like, you can download for free

http://bama.edebris.com/manuals/hp/8558b/
 
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