# Need help designing a High Frequency Amplifier

#### Newbie_01

Joined Feb 26, 2019
12
Hi guys,

I wanted to desgin a HF Amplifier. The main quest is to design it with the used diode and inductors. My problem: At the moment the Amplifier does not produce an output. I am measuring where you can see the probe and in this point I have an offset but no amplification (see attached image). Anyone knows why ? So basically I get a signal input of 50mV with a frequency between 80-110 kHz. I need to amplify it 20db so the output voltage should be 5v. Anyone knows what to change that my offset goes down to zero and my output signal goes up to 5 Volts ?

Any help appreciated.

Thanks.

#### drc_567

Joined Dec 29, 2008
1,156
... just looking at the BFR182 datasheet ... the DC voltage at the collector is listed at 8 volts for the nominal hfe specification of 100. The DC voltage out of Rvar2 seems about right, but is down to 4 something at the Q2 collector ... so maybe adjust Rvar2 a little higher.
... the base voltage of Q2 would have to be something like 0.65 volts for Q2 to have some reasonable value of collector current.
... also, where is your load? ... kind of looking for a resistor, but don't see anything at the Q2 collector branch.
... mostly guessing here, so no sudden moves, and hopefully no smoking parts.

#### sghioto

Joined Dec 31, 2017
2,703
The voltage at the junction of R7 and R8 is too low. Increase the value of R7
SG

#### MrChips

Joined Oct 2, 2009
25,022
80 -110 kHz would not be considered as being high frequency.
I think TS meant 80 - 110 MHz.

What do you mean by amplify to 20dB, some absolute voltage output or a voltage gain of 10?

20dB is 10 times voltage ratio.
20dB is 100 times power ratio.

#### sghioto

Joined Dec 31, 2017
2,703
Actually the TS needs to amplify it 40db to get 5 volts from 50mv.
SG

#### Newbie_01

Joined Feb 26, 2019
12
... just looking at the BFR182 datasheet ... the DC voltage at the collector is listed at 8 volts for the nominal hfe specification of 100. The DC voltage out of Rvar2 seems about right, but is down to 4 something at the Q2 collector ... so maybe adjust Rvar2 a little higher.
... the base voltage of Q2 would have to be something like 0.65 volts for Q2 to have some reasonable value of collector current.
... also, where is your load? ... kind of looking for a resistor, but don't see anything at the Q2 collector branch.
... mostly guessing here, so no sudden moves, and hopefully no smoking parts.

Thank you. I think my load will be a parallel 1M Ohm resistor of an oscilloscope.

#### Newbie_01

Joined Feb 26, 2019
12
Actually the TS needs to amplify it 40db to get 5 volts from 50mv.
SG

Exactly, but I need to get 5 V pp from 50mv pp. When I increase the value of R7, i am just getting a lower offset, so I think DC amplification. The AC curent stays the same 50mV pp.

#### Newbie_01

Joined Feb 26, 2019
12
80 -110 kHz would not be considered as being high frequency.
I think TS meant 80 - 110 MHz.

What do you mean by amplify to 20dB, some absolute voltage output or a voltage gain of 10?

20dB is 10 times voltage ratio.
20dB is 100 times power ratio.

You are absolutly right. I am sorry. The frequency range is between 80-110 Mhz.

I need to amplify a 50mV pp AC signal to a 5V pp AC Signal with no offset. So from -2.5V to 2.5V with the arithmetic mean of 0.

#### drc_567

Joined Dec 29, 2008
1,156
Don't be concerned about Rvar2 just yet.
... the immediate problem seems to be that the DC voltage measured from the base to the emitter of Q2 must be increased to at least 0.65 volts. Presently, that voltage shows to be only 438 mV. One possibility of doing this is to increase R7, as mentioned by sghioto previously. So try using a larger value of R7 ... maybe double it or whatever you have in that range. .... Then take another measurement of the Q2 base to ground DC voltage and see if it us in the range of 0.65 to 0.7 volts DC. This is the active range for NPN transistors. At 0.4 volts, Q2 is turned off and has no collector current.
... One other item to fix is placing a load resistor at Q2. Typically, amplifiers require a matching load impedance in order to operate properly. Usually, this would be a power resistor of 50 ohms. Try placing a 50 ohm 1/2 watt resistor at terminal 2 of C13 between that capacitor and ground. This terminal would be the location of the output voltage, so see what the AC voltage waveform looks like there.
... Increasing Rvar2 is counter-productive ... actually reducing it would cause an increase in voltage at Q2 ...Try gradual changes.

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#### Newbie_01

Joined Feb 26, 2019
12
Don't be concerned about Rvar2 just yet.
... the immediate problem seems to be that the DC voltage measured from the base to the emitter of Q2 must be increased to at least 0.65 volts. Presently, that voltage shows to be only 438 mV. One possibility of doing this is to increase R7, as mentioned by sghioto previously. So try using a larger value of R7 ... maybe double it or whatever you have in that range. .... Then take another measurement of the Q2 base to ground DC voltage and see if it us in the range of 0.65 to 0.7 volts DC. This is the active range for NPN transistors. At 0.4 volts, Q2 is turned off and has no collector current.
... One other item to fix is placing a load resistor at Q2. Typically, amplifiers require a matching load impedance in order to operate properly. Usually, this would be a power resistor of 50 ohms. Try placing a 50 ohm 1/2 watt resistor at terminal 2 of C13 between that capacitor and ground. This terminal would be the location of the output voltage, so see what the AC voltage waveform looks like there.
... Increasing Rvar2 is counter-productive ... actually reducing it would cause an increase in voltage at Q2 ...Try gradual changes.

Okay, done - and yay... an amplificated signal. Thats great! At the moment it is 7x Amplification. Now I just need to get to 100. I had to replace the transistor because this is the easiest to get for me. But this shouldn´t make a difference. Se atteched images.

#### drc_567

Joined Dec 29, 2008
1,156
... seems to be an improvement.
This particular transistor requires 50mA collector corrent, and a collector to emitter voltage of 9 volts DC. ... So, try adjusting the base voltage slightly higher ... 20 or 30 mV .... millivolts .... could make the difference, and see if the amplification increases.
... Also, Vce on the transistor needs to be higher ... 9 volts. See if decreasing Rvar2 slightly improves this number.

edit ... just now looked at current gain ... hfe ... seems to be right at max value ... 104.
For right now, just try getting Vce up to about 9 volts DC.

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#### drc_567

Joined Dec 29, 2008
1,156
... It looks like 3 volts DC is being dropped across R9. ... can't specifically say what to do to improve the Q2 Vce number. If you reduce R9, you will probably cause the Q2 Vbe value to increase too much. It looks like this may require some recursive ... trial and error ... adjustment in order to get everything right.

#### drc_567

Joined Dec 29, 2008
1,156
... maybe if Rvar2 is decreased slightly, Q2 Vce will increase. However Q2 Vbe will also increase, so try decreasing R7 in order to bring Vbe down a little. ... at some point, everything will be just right.

#### Newbie_01

Joined Feb 26, 2019
12
Hi, yes, I have experienced that too. I have deleted R2 and set Rvar2 to 1. I have tried lots of possibilities and the best I got was 1Vpp with this Circuit:

#### drc_567

Joined Dec 29, 2008
1,156
... it looks like C12, at the Q2 collector branch is adjustable. Try adjusting C12 ... one way or the other, and see if the output amplitude is improved.

#### drc_567

Joined Dec 29, 2008
1,156
... The Vbe of Q2 is exessive. If it cannot be constrained to a range between 0.65 and 0.7 volts DC, the transistor will not be amplifying. Vce at Q2 is slightly high, so increase Rvar2 just a little.
... If you put a smaller resistor in for R7, Vbe will decrease. Try going back to 120 ohms for R7.

#### Newbie_01

Joined Feb 26, 2019
12
Okay, hmm AFAIK C12 is only for stabilization purposes. So I changed it values, but I do not got more / less amplification. I just lowered R7 to get to a Vbe in the right range. Now I am getting only a 600mVpp output.

#### drc_567

Joined Dec 29, 2008
1,156
... Vbe is just about right, but Vce is 12 or so and it should be 9.

#### drc_567

Joined Dec 29, 2008
1,156
... another spec from the datasheet:
The ratio of the DC collector current to the DC base current is the current gain, known as hfe. The datasheet value of hfe is given as 80, with Ic at 50 mA, and Vce at 9 volts DC.
... not quite sure how to increase Ic to 50 ma ... hfe seems to be already at maximum.

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