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I want to design a common emitter amplifier with these specifications:

  • Av (gain) = 50
  • VCC = 5 V
  • Rload = 0.25 kΩ

Maximum symmetrical swing is considered but I can not get the collector and emitter resistance values ​​correctly.

Transistor
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ghazaleh moghadam
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    Can you show us your topology and what you have tried to solve for the necessary resistances? As it stands, this looks like a homework question with no detail or shown attempt, and as such, is subject to being closed under site policy. – nanofarad Sep 23 '20 at 19:53
  • What is wrong with the results you're getting? And add the schematic to your question. –  Sep 23 '20 at 20:00
  • @ghazalehmoghadam Are you seriously supposed to just use one BJT for a gain of 50 into a 250 Ohm load and using only a 5 V supply? Or is this a more general question and not limited to this specific type of CE amplifier? – jonk Sep 23 '20 at 21:56
  • this is a basic circuit you can research in any general electronics book on the first 20 pages... come on, google suddenly stopped working? – schnedan Sep 23 '20 at 22:06
  • @jonk no it's not general question i need to design the amplifier with these specifictions. Maybe i should use multi stage amplifie? What is your suggestion? – ghazaleh moghadam Sep 24 '20 at 08:23
  • @ghazalehmoghadam Yes, multi-stage would be the idea. And, with such a low load I might consider a 2-quadrant output driver stage running class-AB. – jonk Sep 24 '20 at 09:22
  • @ghazalehmoghadam Look, with output impedance of about $250:\Omega$, you'll want your own source impedance (driver impedance) to be about a tenth of that or less. So less than about $25:\Omega$, or so. You can't do that readily with a CE stage alone. This reaches well into the area where you want a 2-quadrant class-AB (or class-A) output stage preceded by the needed voltage gain in another stage or so. You are looking for about 34 dB of voltage gain. This is also going to be a problem for limiting higher frequencies, too. See this. – jonk Sep 25 '20 at 04:39

1 Answers1

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To get a gain of 50 with a collector load resistance of 250 ohms requires (on the face of it) an emitter resistor of 5 ohms but, this is where things start to get tricky. For any BJT, \$r_E\$ (the internal emitter resistance) needs to be lower than maybe 1 ohm to get anything like a reliable gain of 50 (with an external emitter resistor of 4 ohms).

To find \$r_E\$ at ambient temperature, it is 26 mV divided by Ic so, to get that down to 1 ohm requires an Ic of 26 mA. Then, if you calculate how to get 26 mA from a 5 volt rail you need a resistance of 192 ohms and, of course, that is less than the collector load resistor so it's a no-go situation.

However, you could try and simulate it with virtually no emitter resistor to see how things shape-up but, for sure, the output distortion will be sub-optimal to say the least and the amplifier gain will be very temperature dependent.

Andy aka
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