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1.5V powered common-emitter amplifier with high gain (200) question

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phn10

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Hi,

I'm reading a book about AM receiver radio. The author designed a 1.5V low power radio, which used a 2-stage common-emitter amplifier to get the gain of 200 (attached image as AM receiver radio.jpg). The author said 1 amplifier has limtied gain, and to get the 200 gain, he needed to use 2 amplifier stages. I don't understand this point.
Based on my knowledge, the gain of the common-emmiter amplifier is Rc/re, in which Rc is collector impedance, and re is base-emitter internal resistance. So based on this equation, given that the ciruit is biased correctly and Vce > Vce(sat), the gain of 1 amplfiier can definately go above 10, even hundreds or thousands. So instead of use 2 amplifiers, I can design a circuit with the same gain with only 1 amplifier.
Does anyone know what is wrong with my assumption?

Note: I also attach a LTspice simulation of my design (attached image as gain_200.png), the input voltage is 4mV, and output voltage is 800mV, so the simulative gain is 200. I built an actual circuit with the same components, but the gain is only 3!!??? Can anyone speculate what is wrong?

Thanks
 

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  • gain_200.png
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the gain of 1 amplifier can definitely go above 10, even hundreds or thousands.
It can't for a real amplifier, particularly at AM frequencies.

You need to consider
- finite transistor output impedance
- transistor capacitances
- inductor winding capacitance and finite quality factor
- load impedance

The Ltspice simulation only accounts for the first two parameters but assumes ideal inductor and no load.
 
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    phn10

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You haven't load, therefore the gain becomes 200.If you connect a proper load, you will see that the gain will drop down to 3.
 

Thank you all for replying. I think the load impedance doesn't matter. As long as the load has high impedance in compare to inductive load(more than 1 Meg), the inductive load will be the primary factor that contribute to the amplifier's gain. In the image attched below, I added a 1 Meg load, and the gain is still around 200. I tried to add a 10Meg and 100Meg, and the gain is still the same

FvM pointed out that inductor has winding capacitance. This point makes sense because this capacitance will decrease the impdance of inductor, especially in AM frequency range, thus decreasing the gain.
One question I just come up with is: if the amplifier has just resistive load, aka no inductor in the above circuit, do I need to have 2 stages amplifier to get the gain I want (200 to 400)? And if no, in what scenario I need a 2 stage amplifiers?

gain_200_with_load.png
 

I mean the load doesn't matter if it has high impedance in compare to the inductive load.
 

if the amplifier has just resistive load, aka no inductor in the above circuit, do I need to have 2 stages amplifier to get the gain I want (200 to 400)? And if no, in what scenario I need a 2 stage amplifiers?
Think. Without an inductor, load resistor Rc is limited to about 1 kOhm for linear amplifier operation, respectively single stage gain can't be higher that about 20.

I mean the load doesn't matter if it has high impedance in compare to the inductive load.
Yes, "if". I don't know what the application and expectable load impedance is.
 

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