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Amplifiers for weak signals

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mstechca

New Member
How many BJT direct-coupled transistor amplifier stages are needed to pick up very weak signals (less than 1/2 as loud as a soft whisper)? If I should use resistors, please specify where I should put them in the circuit and why.
 

kinjalgp

Active Member
What is your audio source? Is it a tape head or condenser microphone?
If its tape head, then you'll require atleast 2 stages of RC coupled BJT amplifer. Remember DC coupled is normally not used for audio purposes since it affects biasing of proceeding stages.
Resistors will be required to bias the transistor, set its Q-point and put it in active region of operation. Even capacitors are required to which sets the lower cut-off frequency of your amplifier.
For condenser microphone, the gain required will be very high more than 100000 for faithful amplificaion signals in micro-volt range. This can not be practically achieved using mutlistage amplifiers ans noise at this level is very high which will be amplified along with the usdio signals. Also designing more than 2 RC coupled stages is a tedeious job. You require an operational amplifier for this purpose which has very high Common Mode Rejection Ratio (CMRR) which eliminates noise signals to quite large extent.
 

Scubasteve

New Member
Hey,

Depending on your source, you will need x amount of gain to get it to the level that you want. Use the standard non-inverting input and plug in the appropriate feedback resistor.

Usually for this sort of thing, it would be better to use an opamp rather a bjt circuit.

Steve
 

mstechca

New Member
I'm trying to amplify a weak radio signal. I have transistors that accept a maximum of 100ma current. How much current should the transistor accept for radio frequency?
 

pebe

Member
mstechca said:
I'm trying to amplify a weak radio signal. I have transistors that accept a maximum of 100ma current. How much current should the transistor accept for radio frequency?
What frequency radio signal? AM or FM? What do you mean by ' transistors that *accept* 100mA'?
 

Scubasteve

New Member
mstechca,

You need to read up on some basic BJT theory before thinking of doing this properly. The 100mA spec. is one of the least characteristics you should be concerned with.

When designing an amplifier, you should only be concerned initially with what you have and what you need. This will give you a gain rating = output/input. Then you need to see if you need more then one stage in your amplifier.

You need to know what frequency you are amplifying, it is one of the most important specs. you should be worried about. Also, the source impedance would be nice to know or even have an approximation of.

Tell us exactly what you are trying to do.. example.. "I need to amplify a 50mVp-p wave at 90MHz to around 1Vp-p, I am using this as an FM amplifier. The source is a 75 Ohm antenna"

Steve
 

mstechca

New Member
I want to make an FM radio and then convert it into a wireless serial port network card. Then I will use it to access the internet. Ever heard of "Packet radio modem"? I want to make it.

The maximum frequency I need is 460Mhz. The frequency I need to connect for 9600bps is between 450Mhz and 460Mhz. The frequency I need for 1200bps is between 145 and 146Mhz.

I am trying to pick up 102.9Mhz without success. Doesn't current and frequency and volume (amplification) go hand in hand?
 

mstechca

New Member
I clicked on the GIF links for the circuit and I didn't see any circuits. Can you post them or give me another site?
 

Gene

New Member
Perhaps you are clicking on the wrong links. Sentence 2 reads:

Fig 1. The modem schematic. Broken up into four parts for legibility: Figure 1a (42K), Figure 1b (31K), Figure 1c (36K), and Figure 1d (31K).


In this sentence, words like "Figure 1a" are blue in color and are underlined. When you click on the word "Figure 1a" (for instance) you will be directed to a portion of the total schematic diagram for the 9600 baud packet modem. This is the circuit.
 

mstechca

New Member
I clicked on those and I get blank pictures. maybe their site isnt working.
 

Gene

New Member
I wonder if I am the only person who can see these?
 

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