Hello,
I have done an emitter bias circuit with an Re feedback resistor to minimize fluctuations generated by different transistor betas!
Please view the circuit in attachment before I go on!
Now, you can see that I have taken extra care to provide precise components in the circuit so I can match the theory as close as possible with the practical world.
Okay, so, when I measure VCe in the circuit attached below, I get very close to approximately 1.2VDC. So the transistor I am using in this very circuit had a beta of 104 according to my multi meter. As you can see we actually calculated for the real beta in reference to it's circuit being 135.
So I yanked out this transistor which had a beta reading 104 according to my multi meter and plugged in another transistor with a measured beta of 113 according to my multi meter.
With this new transistor, my VCe was 1.018 VDC. So I said to myself,
"okay the Re feedback resistor is having a little trouble holding the VCe at 1.2VDC, But hey it ain't that bad!"
Then I tried a transistor with a measured beta of 156 according to my multi meter and yyyiiiiiiiikes !!!!!!!! I nearly fell off my chair LOL!!!
VCe was a deceptive value of 0.318 VDC..... waaaaaay off even though I used the emitter bias method with an Re resistor!!!!
Now what ????
As you all have pointed out to me, I can't always assume that my transistors will have a beta between say 90 and 115... what happens if I get a transistor with an off the wall beta like this one... why doesn't the Re compensate for this????
Is it because Re has to be higher, lower... confused!!!!
any help is greatly appreciated!
PS. I noticed that the latter transistor was a KSP2222 instead of a PN2222... this should not be a problem ... right?
r
I have done an emitter bias circuit with an Re feedback resistor to minimize fluctuations generated by different transistor betas!
Please view the circuit in attachment before I go on!
Now, you can see that I have taken extra care to provide precise components in the circuit so I can match the theory as close as possible with the practical world.
Okay, so, when I measure VCe in the circuit attached below, I get very close to approximately 1.2VDC. So the transistor I am using in this very circuit had a beta of 104 according to my multi meter. As you can see we actually calculated for the real beta in reference to it's circuit being 135.
So I yanked out this transistor which had a beta reading 104 according to my multi meter and plugged in another transistor with a measured beta of 113 according to my multi meter.
With this new transistor, my VCe was 1.018 VDC. So I said to myself,
"okay the Re feedback resistor is having a little trouble holding the VCe at 1.2VDC, But hey it ain't that bad!"
Then I tried a transistor with a measured beta of 156 according to my multi meter and yyyiiiiiiiikes !!!!!!!! I nearly fell off my chair LOL!!!
VCe was a deceptive value of 0.318 VDC..... waaaaaay off even though I used the emitter bias method with an Re resistor!!!!
Now what ????
As you all have pointed out to me, I can't always assume that my transistors will have a beta between say 90 and 115... what happens if I get a transistor with an off the wall beta like this one... why doesn't the Re compensate for this????
Is it because Re has to be higher, lower... confused!!!!
any help is greatly appreciated!
PS. I noticed that the latter transistor was a KSP2222 instead of a PN2222... this should not be a problem ... right?
r
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