# The Mystery Behind a 20dB Amp at 20MHz

• waht
In summary, the conversation discusses an elusive schematic of a 20 dB amplifier at 20 MHz, with a diode that has 83 ohms resistance when current is flowing through it. The schematic shows a base voltage of Q4 that is higher than the supply voltage, and the purpose of L7 = 22uH and R32 is to provide isolation and negative feedback to prevent oscillation and stabilize the DC operating point. The conversation also mentions that this schematic is a small part of an IF amplifier for an HP 8558b 1.5 GHz spectrum analyzer. The speaker finds the schematic interesting and offers to share it for free.
waht
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.

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

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## 1. What is a 20dB amp?

A 20dB amp, also known as a 20 decibel amplifier, is an electronic device that increases the amplitude or strength of a signal by 20 decibels. This essentially means that the output signal will be 10 times stronger than the input signal.

## 2. What does 20MHz refer to in this context?

In this context, 20MHz refers to the frequency at which the amplifier is operating. This means that the amplifier is designed to work best with signals that have a frequency of 20 million cycles per second.

## 3. What is the significance of unlocking the mystery behind this 20dB amp at 20MHz?

Understanding the inner workings of this specific amplifier can provide valuable insights into the design and functionality of similar amplifiers. It can also help in optimizing the performance of the amplifier and identifying any potential limitations.

## 4. How is the mystery behind this amp being unlocked?

The mystery behind this amp is being unlocked through scientific research and experimentation. Scientists may use various techniques such as computer simulations, laboratory experiments, and mathematical modeling to understand the behavior of the amp at different frequencies and input signals.

## 5. What potential applications does this amp have?

This amp can be used in a variety of applications where a signal needs to be amplified, such as in telecommunications, audio equipment, and medical devices. Understanding its specific characteristics at 20MHz can also help in designing better and more efficient amplifiers for specific purposes.

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