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RF signal amplification

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marcbarker

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:D i'm greek...
You're in good company... there's a self-taught inventor "Magic Alex", he was born in Athens, came to London UK as a student. He originally repaired televisions and became freinds with the Beatles (who looked to him as a electronics Guru), then he worked for rich and famous people doing 'technical' work for them. He always had different ideas. Magic Alex - Wikipedia, the free encyclopedia

i have difficulties understanding this weird biasing with the U2...
biased in class c http://en.wikipedia.org/wiki/Electronic_amplifier#Class_C
 
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whiz115

Member
You're in good company... there's a self-taught inventor "Magic Alex", he was born in Athens, came to London UK as a student. He originally repaired televisions and became freinds with the Beatles (who looked to him as a electronics Guru), then he worked for rich and famous people doing 'technical' work for them. He always had different ideas.
interesting... i didn't knew about him, but i know about another greek also named alex! "Sir Alex issigonis" the designer of the famous english car named Mini cooper.

Alec Issigonis - Wikipedia, the free encyclopedia

here in greece in many instances we don't let our people become successful, instead we take em down mostly due to underestimation of their abilities and consequently lack of support and inertia for their work... that's why they "escape" to foreign countries where they can have the potentials. As greeks We love foreign stuff, we hate what is ours and it is homemade..

but i believe we went far too much offtopic! :D
 

Damo666

Member
Can somebody explain why all the stages except the final need biasing? I was under the impression that class c rf amps are always off until a signal enters. So why forward biasing. I don't undertand.
 

unclejed613

Well-Known Member
Most Helpful Member
i'm thinking you may want a bypass cap from the L3-R3 junction to ground, so all of the RF voltage is developed across L3. R3 is just a current limiter for the transistor
 

whiz115

Member
i'm thinking you may want a bypass cap from the L3-R3 junction to ground, so all of the RF voltage is developed across L3. R3 is just a current limiter for the transistor
if you think it's mandatory i should add it... i did the same question before
but i didn't got any responce just tell me a possible value for that.

i would like to ask what could be the absolute minimum input level of my amplifier so it can amplify a signal?
 

kchriste

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For low level signals at the base (apx 0.7V or less), you need bias or the transistor cannot even turn on.
 
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unclejed613

Well-Known Member
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if you think it's mandatory i should add it... i did the same question before
but i didn't got any responce just tell me a possible value for that.

i would like to ask what could be the absolute minimum input level of my amplifier so it can amplify a signal?
0.01uf (10nf) would be fine...

the minimum input level will depend on what kind of gain you get from the first stage, and what your best impedance match between the two stages gives you at the base of the output transistor, you have to have at least 0,7V positive peak value to turn on the output transistor.. you should also use a voltage divider, rather than the "self-bias" method on the first stage. that way you can adjust the bias on the first stage. the best reason for doing this is that the gain of a transistor varies with collector current, and being able to change the bias (while keeping the transistor within it's class A operating range) will allow you to get maximum gain out of the transistor. this change in gain (Hfe) can be as wide as a 100:1 ratio, depending on the transistor, but between 10:1 and 30:1 is most common. in most transistors, the relationship between Hfe and collector current is inverse, so the closer you get to cut-off, the higher the gain.
 

mneary

New Member
Can somebody explain why all the stages except the final need biasing? I was under the impression that class c rf amps are always off until a signal enters. So why forward biasing. I don't undertand.
You don't forward bias a class C stage; you bias it so that it, as you say, is off until a signal enters. Perhaps the term bias is confusing. It doesn't just mean turning the transistor on - it means setting an operating point.

For example, an inductor from base to emitter sets the bias to zero, so that only the AC input voltage greater than the base threshold can turn the stage on. Bias can also be negative.
 

unclejed613

Well-Known Member
Most Helpful Member
that's why i didn't say "bias"..... maybe a more accurate term would have been "drive it into conduction". to add any positive bias would make it a class B amp or a class A amp. adding negative bias would improve the efficiency of the amp, but would require more drive signal. it's best left at zero bias. the first transistor is running class A, and picking a good operating point for it will maximize the gain, and that's why i didn't make an offhand prediction of how much minimum signal is required at the input to drive the output transistor into conduction. i don't like using transistors in self biasing configurations, because normal device-to-device variations in Hfe and Vbe make self biased circuits inconsistent. better to set a stable operating point and minimize the effect of random variations in Hfe and Vbe.
 
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