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I build a common emitter amplifier based on BFP420 BJT, and would like to measure its gain and input and output impedance. As the gain should be within 25dB, so I connected a 40dB 50 ohm input/output attenuator between CH0 and the input of the amplifier to avoid driving the amplifier into saturation, then connected the output of the amplifier to CH1.

The negative dBs readings of S21 were making sense to me after adding up logically 40dB to come up with the actual amplifier gain. However as the inserted attenuator impedance at CH0 is 50 ohm, the readings of S11 was showing as almost flat 50 ohm across the frequency range which is expected.

To measure amplifier input impedance (S11), I removed the attenuator and connected amplifier input directly to CH0 and connected amplifier output to a 50 ohm load. As the amplifier is driven into saturation, I am not sure if the S11 readings are correct, and I can rely on for small signal.

I repeated the same but swapping the amplifier input and output to measure the output impedance (S22).

Is this the right way to measure input and output impedance for such amplifier.in the absence of power control on the nanoVNA? Is there a better way?

There is limited output power control on the nanoVNA you can use (limited, no 30dB control)
Use a terminal emulator to connect to the nanoVNA and you can use the "power" command to 4 levels of output power
power 0
is the lowest level,
power 3
the highest level
The levels result in 2, 4, 6 and 8mA into 50 ohm so the maximum current range you have is 8 / 2 = 4 , 4 times 4 (as you want the power range, not the mA range) gives you a factor 16 in power range

But for you amplifier this range is insufficient so I think you are using the right approach

W.r.t the negative reading, if you do a calibration of the thru with the 40dB attenuator in line (no amplifier) the new 0 level of S21 will help showing positive levels for you amplifier.
By not storing the updated calibration it is a real temporary measurement calibration and when the nanoVNA is reset everything goes back to normal.
Whether the saturated measurement of S11 makes sense depends on the type of input component in your amplifier.

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Erik, PD0EK

Hi aa_Talaat
Sort of right.
For the input impedance measurements you may calibrate Ch0 on the output side the inserted attenuator.
40dB is a bit tough as it will introduce larger measurement error/noise than if you used a 20dB attenuator which might not saturate the amplifier. I you have an oscilloscope/small signal power meter you may inspect the output or if you have more attenuator what the S21 output with 40dB versus 20dB in the TX path and if difference is 20dB then no saturation.
Kind regards
Kurt

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Sendt: 28. november 2019 12:21
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Emne: [nanovna-users] Measure S11 and S22 for active devices

I build a common emitter amplifier based on BFP420 BJT, and would like to measure its gain and input and output impedance. As the gain should be within 25dB, so I connected a 40dB 50 ohm input/output attenuator between CH0 and the input of the amplifier to avoid driving the amplifier into saturation, then connected the output of the amplifier to CH1.

The negative dBs readings of S21 were making sense to me after adding up logically 40dB to come up with the actual amplifier gain. However as the inserted attenuator impedance at CH0 is 50 ohm, the readings of S11 was showing as almost flat 50 ohm across the frequency range which is expected.

To measure amplifier input impedance (S11), I removed the attenuator and connected amplifier input directly to CH0 and connected amplifier output to a 50 ohm load. As the amplifier is driven into saturation, I am not sure if the S11 readings are correct, and I can rely on for small signal.

I repeated the same but swapping the amplifier input and output to measure the output impedance (S22).

Is this the right way to measure input and output impedance for such amplifier.in the absence of power control on the nanoVNA? Is there a better way?

This is a tricky case as the power out can overload the device.

The solution I use is attenuate the output, run S21 and use an
external return loss bridge with an amplifier on the return port
to the channel 1 input. With known gains and attenuation its
easy to get return loss, input impedance, and then input SWR

Same methods used for receiver inputs, with care to insure
the input is below overload.


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As previously posted, you will need to add a suitable 50 ohm pad to both CH0 and CH1 and do a full calibration with the pads in place. Hence, you must take care not to drive the DUT into compression (monitor bias current), the bias current should not change. As well, you must take care not to drive the vna into compression at CH1. The level of pad unfortunate limits your measurement dynamic range. However, if this is an unmatched device and the measurement frequency is well below the F _tau of the device, that is not an issue, s11 and s22 will be close to unity and most likely capacitive.

You do not mention, how are you applying bias to the DUT. Are you using a bias tee. It may be part of the measurement unless you built a really great bias tee. That is to say it is transparent at the measurement frequency. If not, then it is part of the measured device.

I strongly suggest you do your measurement with a device for which you have data. Measure at a low frequency, say HF and a single device that has a reasonable S21, not to large, say 12-to-15 dB. That way you can minimize some of these complications. I have measured and compared simple 2N3904 devices at Vce of 6 to 9 volts and Icq of 1-5 mA. Small signal s data is available by using LT Spice as the device Spice models are in the program. And Spice provides all the S data output easily. You will also have real bias networks in there or you can put in non ideal bias tee networks and observe the results in Spice. Then go to the vna and do the same. The results will match if you did all the steps correctly!

Alan

For S11 you can connect CH0 to input and dummy load on the amplifier output.

For S21 you can use 20-30 dB attenuator on the input and 20-30 dB attenuator on the output of amplifier. You can calibrate NanoVNA with attenutators or just add sum of attenuators value to result. Attenuator value will depends on your amplifier. Just note that you're needs to fit amplifier output to -10 dBm max.

For S21, the good choice for output attenuator is equal to amplifier gain. And for input attenuator the good value is 20-30 dB attenuator.

For example, if your amplifier has 40 dB gain, then use 20 dB attenuator on the input and 40 dB attenuator on the output.

This is all fine for accessing the mag of s21. However, I also need its phase. I have not gone through the arithmetic but my concern is diluting the phase term. Dynamic range and a degree of uncertainty may render angles in jeopardy.

Alan