Basic transistor circuits

Thread Starter

Zeeus

Joined Apr 17, 2019
600
Hi..Trying to understand

First image : Please explain more
Transistors are 2n3904 and 2n3906

Anyways, I added this cap.. 2nd image : replaced the 6k (6.2k) with 4.7k to get 6v at collector...Gain : Rc/re around 240..re changes yeah but the gain I observed was about 50..why so??

3rd image is same as first but with emitter resistors and did not change the 6.2k..6v at collector of Q1...Changing the cap (100k) to 4.7k put's collector of q1 at 7.4v..changing it to 1k sends collector to about 11v..The capacitor is there to take away current from the base?

4th image : R4 and R7 are 150...If I remember correctly (not sure i checked : think i did), voltage at collector of q2 was about 6v... I used an IC with transistors CA3096...For the inputs, please look at 5th image : Is this okay to do? don't want to destroy my generator..So doing this get's the 6th image

Now one input to the differential amp was 1khz and the other was the sum of the two frequencies (1khz and 140hz)....7th image was the result

when I changed the inputs (1khz to the other side of amp and the sum of frequencies input to the other side)..8th image was result : was expecting this since the difference between the two inputs is the 140hz signal : This is how this amp should work right? but notice that the (sum of frequencies input) has changed shape

The 9th image : pnp transistors.I added the base bias resistor (not shown in image)....When I connected the signal input (I did not turn it on)..Connected scope probe to the input which is not turned on : The last image is the result : like lc ringing : what's happening?


Thank you thank you thank you
Just got LTSpice :)
 

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ericgibbs

Joined Jan 29, 2010
11,091
hi Zeeus,
I would suggest that you post a copy of one of the LTS 'asc file ' problem circuits and we can work thru the remaining 8 different options, one at a time,.
Your Thread is confusing to read.
E
 

Thread Starter

Zeeus

Joined Apr 17, 2019
600
Someone please reply on this : back to it soon

I made a similar circuit with +-12V...Why are the capacitors that way? because the (-) side of caps are below 0v? How to know polarity?

Also, for the input..I used the 2nd image to create input to the first image (diff amp). Input A is from the left source and input B is at the top of both 3k resistors (this top is the sum of the two inputs from generator)....Generator 1: 1khz generator 2 : 140hz...

input 1 is 1khz..input 2 is (140hz + 1khz) correct? 3rd image

so using diff amp should get the 140hz?

output is yellow signal

When I used the two inputs to the diff amp : I got 4th image..input 1 at input x of amp and input 2 at input y of amp (x and y : bases of amp)

I changed the inputs : input 1 at input y and input 2 at x and got 5th image which is 140hz but low amplitude (input 2 - input 1 should give the 140hz yeah?) but if you look, the two inputs are almost the same frequency

Please try a bit to understand and assist. Thanks
 

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Jony130

Joined Feb 17, 2009
5,186
Why are the capacitors that way?
Notice how the base current is flowing and the voltage polarity across 10kΩ resistors.

When you are using asymmetrical input of a differential amplifer you no longer have a true differential amplifier.
If you are using the left input. you now have an emitter follower + common base amplifer.
 

Thread Starter

Zeeus

Joined Apr 17, 2019
600
Notice how the base current is flowing and the voltage polarity across 10kΩ resistors.
It's a bit less that 0 (volt across 10k)..Okay but look at the attached image : Even with voltage divider as bias (not shown), the author uses the cap with negative side to divider and positive side to input..

When you are using asymmetrical input of a differential amplifer you no longer have a true differential amplifier.

If you are using the left input. you now have an emitter follower + common base amplifer.
don't understand :( Anyways, don't worry : by the end of the day will understand slightly better

can't attach image
 

Jony130

Joined Feb 17, 2009
5,186
It's a bit less that 0 (volt across 10k)..
The situation will look like this:

1.PNG


I assumed I_tail ≈ 15V/7.5kΩ = 2mA and β = 100. And electrolytic capacitors can withstand up to around 1.5V of reverse voltage without blowing up.


Okay but look at the attached image : Even with voltage divider as bias (not shown), the author uses the cap with negative side to divider and positive side to input..
I suspect that this is just an error on the schematics. And of course, it is incorrect, do not try it at home.
 

Thread Starter

Zeeus

Joined Apr 17, 2019
600
The situation will look like this:

View attachment 176818


I assumed I_tail ≈ 15V/7.5kΩ = 2mA and β = 100. And electrolytic capacitors can withstand up to around 1.5V of reverse voltage without blowing up.




I suspect that this is just an error on the schematics. And of course, it is incorrect, do not try it at home.
LOL thanks
 

BobaMosfet

Joined Jul 1, 2009
1,224
Hi..Trying to understand

First image : Please explain more
Transistors are 2n3904 and 2n3906

Anyways, I added this cap.. 2nd image : replaced the 6k (6.2k) with 4.7k to get 6v at collector...Gain : Rc/re around 240..re changes yeah but the gain I observed was about 50..why so??

3rd image is same as first but with emitter resistors and did not change the 6.2k..6v at collector of Q1...Changing the cap (100k) to 4.7k put's collector of q1 at 7.4v..changing it to 1k sends collector to about 11v..The capacitor is there to take away current from the base?

4th image : R4 and R7 are 150...If I remember correctly (not sure i checked : think i did), voltage at collector of q2 was about 6v... I used an IC with transistors CA3096...For the inputs, please look at 5th image : Is this okay to do? don't want to destroy my generator..So doing this get's the 6th image

Now one input to the differential amp was 1khz and the other was the sum of the two frequencies (1khz and 140hz)....7th image was the result

when I changed the inputs (1khz to the other side of amp and the sum of frequencies input to the other side)..8th image was result : was expecting this since the difference between the two inputs is the 140hz signal : This is how this amp should work right? but notice that the (sum of frequencies input) has changed shape

The 9th image : pnp transistors.I added the base bias resistor (not shown in image)....When I connected the signal input (I did not turn it on)..Connected scope probe to the input which is not turned on : The last image is the result : like lc ringing : what's happening?


Thank you thank you thank you
Just got LTSpice :)
This is a glaring example of how electronics is mistaught. Look at the description in image one. 'base bias resistor'. Is that correct? Yes of course. Is it clear to a newbie? Absolutely not. Instead, it should be worded thus 'weak pull-down resistor to hold the base at ground in the absence of an overriding signal on the base'.
 
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