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transistor question

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defcon31

New Member
hello,

as shown on the figure below, in the first case when i set a 0-5V pulse on input of a transistor, i get a 0-24V pulse on the output, as expected...

but in the second case in the figure, when a attach a load, the voltage drops to 4V on the output... what is the problem??? how can i fix that?

how can i fix it?

thanks in advance
 

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samcheetah

New Member
defcon31 said:
hello,

as shown on the figure below, in the first case when i set a 0-5V pulse on input of a transistor, i get a 0-24V pulse on the output, as expected...

but in the second case in the figure, when a attach a load, the voltage drops to 4V on the output... what is the problem??? how can i fix that?

how can i fix it?

thanks in advance

the problem is that the total load has changed. so eventually the voltage drops. in the first case the total load was only the collector resistor while in the second case the total load is both the collector resistor and the new load that u have added. thats why the voltage has decreased.

to get rid of this problem couple a capacitor at the output stage of the amplifier i.e before the load add a capacitor in series. the bias values will remain the same as they were before. and u should also add a coupling capacitor in the input stage of the amplifier.
 

Russlk

New Member
While Samcheetah's solution may work if the AC impedance of the load is higher than the DC, a better solution is to use an emitter follower:
I hope you have a current limiting resistance at the input base, otherwise there is danger of blowing the base-emitter junction.
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
samcheetah said:
hey thanx i didnt know that the difference in AC and DC impedance is considered when coupling capacitors. the emitter follower is a nice thing in that it eradicates this problem.

It all depends on what you are doing, you didn't mention resistor values, or what the load was you are trying to drive.

Your original circuit can only 'sink' current (through the transistor), the 'source' current is only that available through the collector load resistor. The last circuit can only 'source' current, through the emitter follower, it can't 'sink' anything at all.

While this doesn't affect your circuit the way it's drawn, it's something to bear in mind - particularly as you could possibly simplify things by replacing the collector resistor in your circuit by the load - saving one resistor, rather than adding an extra transistor.
 

samcheetah

New Member
It all depends on what you are doing, you didn't mention resistor values, or what the load was you are trying to drive.

i didnt have to mention anything because it wasnt my question

anyway the explanation u gave was very good. the source sink analogy helped me understand the idea. im not sure about replacing the collector resistor with the load. i havent seen that type of a configuration in any book. although it is a nice idea[/quote]
 

Styx

Active Member
i didnt have to mention anything because it wasnt my question

anyway the explanation u gave was very good. the source sink analogy helped me understand the idea. im not sure about replacing the collector resistor with the load. i havent seen that type of a configuration in any book. although it is a nice idea


I do that all the time as I do 3ph PWM inverters using IGBT's. A quick google came up with this - ok nothing abt what you want but as you see the load is in the collector leg

2304-diopro.gif
[/quote]
 
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