A very simple single npn transistor setup but doesn't work (pic included) ?

Thread Starter

dangky

Joined Nov 3, 2017
5
This is the setup for C1815 npn transistor:


Input wave form:


Wave form at base:


Wave form at collector:


The C1815 transition freq is 80 MHz, so it should work at 1.7 MHz right ? Why can't it turn on and off at all ? It looks like it always conduct. Is it because of input capacitance ?
 

KeithWalker

Joined Jul 10, 2017
451
This is the setup for C1815 npn transistor:


Input wave form:


Wave form at base:


Wave form at collector:


The C1815 transition freq is 80 MHz, so it should work at 1.7 MHz right ? Why can't it turn on and off at all ? It looks like it always conduct. Is it because of input capacitance ?
Hi Dangky, the information you posted is a bit limited. Are your power supply negative, the transistor emitter, the signal source common and the scope common all connected together? If not, that will cause your problem.
 
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Thread Starter

dangky

Joined Nov 3, 2017
5
Hi Dangky, the information you posted is a bit limited. Are your power supply negative, the transistor emitter, the signal source common and the scope common all connected together? If not, that will cause your problem.
I'm sure I connected the power supply correctly. And they have a common ground of course.
 

AlbertHall

Joined Jun 4, 2014
8,504
Further, you show an input waveform of about 3V peak to peak, but what is the actual voltage level at the bottom (or top) of that waveform?
 

AlbertHall

Joined Jun 4, 2014
8,504
Those voltages don't make sense.
It is what you would see if the 5V was connected to the resistor on the base and, maybe, the input was connected to the collector.
Double check the transistor type, and its connections according to the datasheet, and the connections to power supply and input signal.
 

dl324

Joined Mar 30, 2015
9,111
Why can't it turn on and off at all ?
Apply 5V to the base resistor with the emitter grounded and the collector connected to 5V with the 1k resistor. Report the collector voltage.

Then ground the base resistor and report the collector voltage.
 

absf

Joined Dec 29, 2010
1,942
Is this how you connected your circuit and took your measurements?
What's the nature of the input signal? Are you intending to amplify the signal or clean up the spites?

cct7.PNG
Allen
 

Thread Starter

dangky

Joined Nov 3, 2017
5
Those voltages don't make sense.
It is what you would see if the 5V was connected to the resistor on the base and, maybe, the input was connected to the collector.
Double check the transistor type, and its connections according to the datasheet, and the connections to power supply and input signal.
Yeah it doesn't make sense, that's why I'm asking. I double checked everything.

Is this how you connected your circuit and took your measurements?
What's the nature of the input signal? Are you intending to amplify the signal or clean up the spites?
Yes the diagram is correct. I want to amplify it (npn as a switch).

I divided the frequency by 16 and checked again. Here is the result. Note that my scope has only 1 channel, so wave forms aren't captured at the same time, but aligned by triggered time. From top to bottom is collector, base, input voltage.



You can see that when input voltage came down to zero, base voltage stayed at 0.7v for some time. So with higher frequency, base voltage is always at 0.7v is understandable.
What I don't understand here is why base voltage took such long time to go down.
Can anyone with a oscilloscope do the same experiment and verify it for me please ? Maybe my scope or npn transistor is bad quality ?
 

Thread Starter

dangky

Joined Nov 3, 2017
5
Well I did some search and saw the baker clamp, and why I need it. So to increase the working frequency I need more part.
I will look into fet transistor to see if the diagram is more simple.
 

absf

Joined Dec 29, 2010
1,942
NPN amp.PNG
Here's the sim in proteus. With 10K resistors the output was badly distorted becoming sawtooth wave.

I am not able to sim 1.6MHz with analog so I use digital clock input but has to bring it down to 3V using a voltage divider. Not sure if 2SC1318 is better than BC108 or not....:cool:

Allen
 
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