Hi,
I do not think any conventional Diy equipment will allow one to measure -3db @ >=100kHz.
However if you compare identical voltages before and after the amplifier them you can measure accurately.
What you need is to accurately measure the gain of the amplfier. Subtract 3db from this gain and build an attenuator to match this calculated gain. Now add a second -3db attenuator in front of the previous.
You can apply a low frequency (500Hz or so) signal to the string of -3db + -Xdb + amplifier and measure the input voltage and output voltage they should be identical (+-0.5%) if you have measured , calculated and built accurately.
Now take the -3db out of circuit (switchable is better) and the amplifier output should have increased by +3db (input=1000mV output=1414mV+-7mV). Increase the test frequency comparing input and output signals and when they are equal you have found the frequency for -3db, fairly accurately. Make sure you have a lowish inductance load on the amp and keep the signal levels low to prevent component overheating. This procedure takes time and the amp is going to be running at 100s kHz for quite a few minutes.
Best of all, build a switchable attenuator block with accurately set steps into a standard impedance (I use Tattenuators with 600r Zin & Zout and 20db,20db,10db,5db,2db,2db,1db,0.5db,0.2db,0.2db,0.1db,0.05db steps into 600ohms, this allows 0db to -61db in 0.05db steps. I then add a 600r load to the attenuator and feed this into the amplifier. The amp sees 300r as source impedance)
I do not think any conventional Diy equipment will allow one to measure -3db @ >=100kHz.
However if you compare identical voltages before and after the amplifier them you can measure accurately.
What you need is to accurately measure the gain of the amplfier. Subtract 3db from this gain and build an attenuator to match this calculated gain. Now add a second -3db attenuator in front of the previous.
You can apply a low frequency (500Hz or so) signal to the string of -3db + -Xdb + amplifier and measure the input voltage and output voltage they should be identical (+-0.5%) if you have measured , calculated and built accurately.
Now take the -3db out of circuit (switchable is better) and the amplifier output should have increased by +3db (input=1000mV output=1414mV+-7mV). Increase the test frequency comparing input and output signals and when they are equal you have found the frequency for -3db, fairly accurately. Make sure you have a lowish inductance load on the amp and keep the signal levels low to prevent component overheating. This procedure takes time and the amp is going to be running at 100s kHz for quite a few minutes.
Best of all, build a switchable attenuator block with accurately set steps into a standard impedance (I use Tattenuators with 600r Zin & Zout and 20db,20db,10db,5db,2db,2db,1db,0.5db,0.2db,0.2db,0.1db,0.05db steps into 600ohms, this allows 0db to -61db in 0.05db steps. I then add a 600r load to the attenuator and feed this into the amplifier. The amp sees 300r as source impedance)