Frequency response of 2N3904 is 800 hz!

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cryptosaint777

Joined Jun 1, 2021
22
Hello community,
Today I was testing some 2N3904 NPN bipolar junction transistors, which according to the datasheet, is capable of switching up to 300 MHZ. I do not expect a very accurate response at high frequencies, however, I tested a 5 vold DC square wave with a 50% duty cycle generated by a 555 timer, and the transistor was only getting up to 1.5 VDC with a 30 KHZ square wave. I thought that was strange, and perhaps I had bad transistors, so I went to a local shop and bought some 2N3904 and 2N3906. I got the same results. At 3 KHZ, the transistor gets up to about 3.5 VDC, it only reaches 4.6 VDC at an astonishingly slow 800 HZ!
Does anyone have any suggestions for a better alternative NPN and PNP transistor for switching?
 

Audioguru again

Joined Oct 21, 2019
3,872
You did not post a schematic so we have no idea how you are powering and connecting a 555 to the transistor and which part of the transistor "gets up to" or "only reaches" certain voltages.
 

Thread Starter

cryptosaint777

Joined Jun 1, 2021
22
You did not post a schematic so we have no idea how you are powering and connecting a 555 to the transistor and which part of the transistor "gets up to" or "only reaches" certain voltages.
I am using a Ben Eater style Astable multivibrator using a 555 timer with the output on pin 3. The output goes through a 330 ohm resistor to the base of the 2N3904, with a 5 vdc 2 amp power supply is connected to the collector. I am measuring the output of the emitter using an oscilloscope.

Sorry for the terrible schematic, I tried to draw on my phone.Sketch_20210615_172905.png
 
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Audioguru again

Joined Oct 21, 2019
3,872
Since you did not post a schematic again, we do not know the supply voltage of the 555 or the emitter to ground load resistor of the transistor.
Why use the low power transistor? Its maximum allowed output current is 200mA like a 555 but its performance is poor at and above 100mA.
 

DickCappels

Joined Aug 21, 2008
8,000
I don't recognize a transistor in your sketch.

From what you have told us your transistor is acting as an emitter follower and it will not have any gain. If you are taking the output from the NE555 pin3 you will get a peak voltage of considerably less than the power supply, to that loss you need to add about 0.6 volts drop between the base and the emitter, so in light of that the circuit is giving you the correct peak output voltage.

You should also have a resistor from the emitter to ground.

Most people who draw a schematic to send over their telephone will draw the schematic clearly with pencil or pen on white paper then take a picture of the drawn schematic and send that. Works out much better that way.

1623804290195.png
(Above) unity gain non-inverting emitter follower. Starting place: Emitter resistor 1K. If the leads are long there may be RF oscillations which can be knocked out with a 1k resistor. Remember to place an 0.1 uf ceramic capacitor from Vcc to Ground to reduce the chance of oscillations.

1623804345092.png
(Above) An inverting amplifier with gain. Starting place: base resistor =10k, collector resistor 1k. Remember to place an 0.1 uf ceramic capacitor from Vcc to Ground to reduce the chance of oscillations.
 
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Papabravo

Joined Feb 24, 2006
17,316
That schematic is WORSE than useless for conveying useful information. It's a good thing you don't work for me because you wouldn't after dropping that steaming pile.
A transistor is a current device not a voltage device and it is a much better amplifier of sinewaves than square waves, and we don't use square waves to measure frequency response. This is the way you do it, by sweeping an AC source and measuring the voltage gain at each frequency. 40 dB is a gain of 100 and it goes to ≈ 350 MHz where the gain is 1 or 0 dB.
 

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Thread Starter

cryptosaint777

Joined Jun 1, 2021
22
I have not added the transistor in the circuit because it was described in the post.
What is the difference in doing it like this?
Thanks for the help.Sketch_20210615_175124.png
 
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Papabravo

Joined Feb 24, 2006
17,316
I need one transistor to drive two other transistors, so I need to have the resistor in series.
That is not the way you use a transistor. But, hey if this learning method is working for you -- knock yourself out. Just don't try to pretend you know what you are doing. I'm certainly not convinced.
 

MrChips

Joined Oct 2, 2009
24,651
Firstly, learn to draw circuit diagrams. This is the language we use to convey electronic ideas.

Do not try to draw on a smart phone or tablet.
Draw it on paper with pencil. Take a photo and post the photo.

This is what a simple transistor amplifier looks like.
We do not design amplifiers this way but this is a starting point.


1623806061451.png
 

crutschow

Joined Mar 14, 2008
28,227
The problem is definitely not the transistor, it is your circuit.
A "better" transistor won't make any difference.
And until we see the complete circuit diagram including resistor values and the output load, we are just spinning our wheels and you are wasting everyone's time.
 

Thread Starter

cryptosaint777

Joined Jun 1, 2021
22
Wow, what the freaking crap? I just asked a simple freaking question. And you say, we can't answer because clearly you are wrong. Wow, thanks for nothing. Now I know why this place sucks.
 

Audioguru again

Joined Oct 21, 2019
3,872
The datasheet of an ordinary LM555 or NE555 shows that its output cannot go as high as the supply voltage. With no load current and a +5V supply, its output high is +3.7V. With 3.7V on the base, the emitter goes no higher than 3.1V. You did not say if your 555 is a Cmos one.

You show a resistor in series with the emitter to OUT. This resistor has no value.
With an oscilloscope at the OUT then the transistor has no load.
 
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