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Problem with hf broadband amplifier

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neazoi

neazoi

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Hello, I am refining this amplifier **broken link removed** this time I perform some measurements as I build it.
I have built the input circuit up to T1.
I connect the 50Ohm oscilloscope to the two points of the transformer (not the center tap).

I expected to see inverter sinewave waveforms, but I see inverter distorted waveforms.
Is that normal?

I thought the inverted sinewaves should be of low distortion there for good amplifier operation.
 

neazoi said:
Hello, I am refining this amplifier **broken link removed** this time I perform some measurements as I build it.
I have built the input circuit up to T1.
I connect the 50Ohm oscilloscope to the two points of the transformer (not the center tap).

I expected to see inverter sinewave waveforms, but I see inverter distorted waveforms.
Is that normal?

I thought the inverted sinewaves should be of low distortion there for good amplifier operation.
Click to expand...

Connecting the 50-Ohm load (do you use a capacitor to separate DC?!) to the "live" point of RF transformer is wrong, and distortion can occur.
The secondary winding can be loaded with 50 Ohms if separated from the DC bias components, try to connect the scope there.
If you follow the instruction, your result may be good. Wideband amplifiers are no easy stuff.
Check the ARRL Radio Amateurs' Handbook for other amplifier designs, V-Fet devices are easier to use.
 

chuckey said:
what level were you using?, what form did the distortion take? what voltagge are you using? . Is your drive distortion free?
Frank
Click to expand...

The level of the crystal oscillator driver is about 0.8vpp. The sinewave of the oscillator is clean, however when I connect it to the base of the power amplifier input transistor and connect the scope at the base, it shows a distorted sinewave (more like a distorted triangle like).
I am using 12v.

- - - Updated - - -

jiripolivka said:
Connecting the 50-Ohm load (do you use a capacitor to separate DC?!) to the "live" point of RF transformer is wrong, and distortion can occur.
The secondary winding can be loaded with 50 Ohms if separated from the DC bias components, try to connect the scope there.
If you follow the instruction, your result may be good. Wideband amplifiers are no easy stuff.
Check the ARRL Radio Amateurs' Handbook for other amplifier designs, V-Fet devices are easier to use.
Click to expand...

I have connected the scope at each of the driver transformer side terminals without a series dc blocking capacitor. It is only one turn secondary, so I assume the 50R oscilloscope load should be ok to be connected there.
My oscillator id sefinitely not 50R based, but I do not thing the amplifier input is 50R either.

If I lower the oscillator level then a clean sinewave (low power) appears at the transformer phasing outputs, but most of the times the preamplifier transistor fails to switch on. What is I change the base bias resistors so that it is biased closer to class-a?

My main concern is that:
Will I still be able to get a clean sinewave (correct the sistortion somehow) at the amplifier output, even if the preamplifier (phasing transformer) output is distorted? Or is this distortion "transfered" to the push pull power amplifier?
 

neazoi said:
The level of the crystal oscillator driver is about 0.8vpp. The sinewave of the oscillator is clean, however when I connect it to the base of the power amplifier input transistor and connect the scope at the base, it shows a distorted sinewave (more like a distorted triangle like).
I am using 12v.

- - - Updated - - -



I have connected the scope at each of the driver transformer side terminals without a series dc blocking capacitor. It is only one turn secondary, so I assume the 50R oscilloscope load should be ok to be connected there.
My oscillator id sefinitely not 50R based, but I do not thing the amplifier input is 50R either.

If I lower the oscillator level then a clean sinewave (low power) appears at the transformer phasing outputs, but most of the times the preamplifier transistor fails to switch on. What is I change the base bias resistors so that it is biased closer to class-a?

My main concern is that:
Will I still be able to get a clean sinewave (correct the sistortion somehow) at the amplifier output, even if the preamplifier (phasing transformer) output is distorted? Or is this distortion "transfered" to the push pull power amplifier?
Click to expand...

Can you, please, read the original instruction carefully?

It states that the source and load are 50 Ohms.
Your 0.8 V p-p across 50 Ohms is 13 mW, NOT 0.4 mW as specified. So you are overdriving T1 and wondering about distortion?
You did not follow the instruction, so do not wonder your results are poor.
 

The input transistor is being starved of current, the 22K should be reduced to below 10K, also the 47 ohm resistor is increasing the input impedance, so the input CURRENT is very low, it would be advantageous to have a step up transformer between the input socket and the base , say 10:1. An alternative would be to reduce the 47 ohms, try 33, 22 etc, until the transistor draws at least 10 mA. Measure the voltage drop across the new "47" ohm, and calculate the current.
Frank
 
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