It seems circuit analysis problems on the Allaboutcircuits board are usually homework problems. I thought I'd bring attention to something that's not a homework problem (but could be involved in homework).
I'm sure T N K, steveb and hgmjr will want to look at this.
I've noticed that if you look for information about models for a simple common emitter amplifier on the web you will find that various models use two different values for the base resistance, r∏ (sorry that looks a little funny; the ∏ is too big). Some use β*re and some use (β+1)*re. Which is correct?
Here:
http://academics.vmi.edu/EE_dbarr/E... Circuit Theory/BJT_small_signal_analysis.pps
are some Powerpoint slides with models for various configurations from a book by Boylestad. I'll attach some images from there for the simple common emitter amplifier with no emitter resistor. I'll ignore ro for my discussion.
If you derive an expression for the voltage gain using β*re for the base resistance, you get Av = -Rc/re. According to this, the gain is independent of β, and in fact remains constant even if β goes to zero; this seems wrong.
If the base resistance is taken to be (β+1)*re, then Av = (β/(β+1))*(-Rc/re). Now the gain goes to zero as β goes to zero, as one would expect.
The difference in the gain expressions is small, and for reasonable β they will give essentially the same result.
But, why not use the more exact expression? It's not that much more complicated. And, even though for this simple amplifier, there's not much difference, in some other topologies it's clear that (β+1)*re is the right value to use. I'll show those in subsequent posts.
I'm sure T N K, steveb and hgmjr will want to look at this.
I've noticed that if you look for information about models for a simple common emitter amplifier on the web you will find that various models use two different values for the base resistance, r∏ (sorry that looks a little funny; the ∏ is too big). Some use β*re and some use (β+1)*re. Which is correct?
Here:
http://academics.vmi.edu/EE_dbarr/E... Circuit Theory/BJT_small_signal_analysis.pps
are some Powerpoint slides with models for various configurations from a book by Boylestad. I'll attach some images from there for the simple common emitter amplifier with no emitter resistor. I'll ignore ro for my discussion.
If you derive an expression for the voltage gain using β*re for the base resistance, you get Av = -Rc/re. According to this, the gain is independent of β, and in fact remains constant even if β goes to zero; this seems wrong.
If the base resistance is taken to be (β+1)*re, then Av = (β/(β+1))*(-Rc/re). Now the gain goes to zero as β goes to zero, as one would expect.
The difference in the gain expressions is small, and for reasonable β they will give essentially the same result.
But, why not use the more exact expression? It's not that much more complicated. And, even though for this simple amplifier, there's not much difference, in some other topologies it's clear that (β+1)*re is the right value to use. I'll show those in subsequent posts.
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