How to proceed with this question? I can't think from where to start.![This is only question, no values were given. "[4]" represents marks of the question.](https://file-hippo.netlify.app/host-https-i.sstatic.net/HB6me.jpg)
\$\begingroup\$
\$\endgroup\$
6
-
1\$\begingroup\$ I can make it have a gain of >100, but I think if you make reasonable assumptions and start with the operating point you'll come to a different conclusion. \$\endgroup\$– Spehro PefhanyCommented Feb 27, 2021 at 4:59
-
\$\begingroup\$ I believe that posting question (a) too would help a lot. What shown so far seems nonsense \$\endgroup\$– carlocCommented Feb 27, 2021 at 7:00
-
\$\begingroup\$ @Spehro Pefhany. Max gain 96 at 25 C :-) \$\endgroup\$– Russell McMahon ♦Commented Feb 27, 2021 at 7:13
-
\$\begingroup\$ @carloc Question (a) is not related to this. \$\endgroup\$– xoxocoderCommented Feb 27, 2021 at 8:47
-
\$\begingroup\$ @RussellMcMahon Okay, I'll bite. \$\endgroup\$– Spehro PefhanyCommented Feb 27, 2021 at 20:44
|
Show 1 more comment
1 Answer
\$\begingroup\$
\$\endgroup\$
2
What "could be" the max collector current? Ic = 2.5V /50k = 50 uA
What is Vbe if Ic = 1mA ? (Answer ~ = 600mV)
- you should try to remember this and confirm on your own in any datasheets.
What is this Vbe? 800 mV
So the collector is not longer an ideal (infinite impedance) current sink and is now a saturated switch.
Whenever an analog output is saturated, you no longer have any gain. ( you might see an attenuated asymmetric wiggle here with some 100mVpp input on 800mV)
So what is the voltage gain here? "none"
What is the current gain? (none)
there's actually a loss here if Vbe = 800 mV drives maybe 100mA to > 1A while Ic = 50 uA in making a switch and you need to put in 10x base current to achieve "rated" Vce(Sat)
So you would never use this circuit as shown.
-
\$\begingroup\$ Just for fun: long long long ago I needed a high side switch for a voltage sense resistor divider. I wanted very low switch voltage drop. Now a low Rdson FET would be the obvious choice. Then not so much so. I used a PNP bipolar with. Forced beta close to 1. Current was still small and only present during sampling. Vcesat was far far lower than published typical and constant enough. \$\endgroup\$– Russell McMahon ♦Commented Feb 28, 2021 at 5:40
-
\$\begingroup\$ Essentially a current mirror comparator \$\endgroup\$ Commented Feb 28, 2021 at 6:56