Bipolar transistors and pulse amplification

In summary: The pulses you are seeing are likely due to saturation of the transistor. To avoid this, you can try reducing the input level or gain or by using a transistor with lower saturation potential.
  • #1
fairchild1
5
0
hello
i have been having trouble working with bipolar transistor. i have been taking pulses that at less then 1us long and amplifying them with transistors. how ever if i have a bipolar transistor as part of my amplifier whether it is my inputs are as part of another stage the pulses always are outputted as a >1us time pulse that has been amplified. i have used the bipolar transistor in may ways to try to eliminate the problem. but they all have the same outcome. the output is always a fast rise and a slow drop which would indicated current flowing easier one way. so i use pnp and a npn to see if the shape would change but it does not. can anyone give me advice.
 
Engineering news on Phys.org
  • #2
Sounds like a saturation problem. Show an example of the circuit you're using and I'll show you how to fix it.
 
  • #3
Does your transistor saturate while amplifying the pulse? If so, try reducing the input level or gain to keep it out of saturation.
 
  • #4
attached it my circuit. it is a standard inverting amplifier.
 

Attachments

  • circuit.bmp
    240.1 KB · Views: 671
  • #5
Can you tell us the source impedance, the load impedance and the high and low voltages of the pulse at the input?
 
  • #6
as an input of the amplifier the charge pulses not voltage pulses so the duration of the pulse is the time it take for the charge to go from source to ground. in addition the voltage is dependent on the resistance to group so the higher the resistance the greater the voltage and greater the time of the pulse. these pulse are from a PMT.
 
  • #7
You need to prevent the transistor from saturating by limiting the base current.

You can use either of the two fixes shown. They both work by shunting away excess base current as the transistor nears saturation. They both require that there is a some amount of source resistance present so as to limit the available drive current, hence the extra resistor may or may not be needed depending on the impedance of whatever is currently driving the circuit.
 

Attachments

  • circuit_fix1.jpg
    circuit_fix1.jpg
    12 KB · Views: 566
  • circuit_fix2.jpg
    circuit_fix2.jpg
    12.3 KB · Views: 620
Last edited:
  • #8
i saw that before how ever i can not increase the impedance of the input other wise i would be increasing the the time of the pulse but i could use it as a second or thirds stage of the amplifier. so i will try it out
 
  • #9
fairchild1 said:
i saw that before how ever i can not increase the impedance of the input other wise i would be increasing the the time of the pulse but i could use it as a second or thirds stage of the amplifier. so i will try it out

In that case then you probably don't need the series resistance so just add the clamp diodes, they will not greatly alter the input impedance of the circuit shown.
 
  • #10
by any chance do you know what the diodes i should use
 
  • #11
I gave example part numbers on the diagrams (1n1418 and bat85)
 
  • #12
I have attached a diagram of what should be happening with your amplifier.

Because it is an inverter, the input pulse duty cycle is inverted and you will get a negative-going pulse as your output if you have a positive-going pulse as your input.

So, the positive-going output pulse could be a lot wider than the positive-going input pulse. I wonder if this is what you are seeing?

Maybe you could modify this diagram to show what effect you are actually getting?
 

Attachments

  • pulse inversion.PNG
    pulse inversion.PNG
    1.6 KB · Views: 620
Last edited:
  • #13
fairchild1 said:
as an input of the amplifier the charge pulses not voltage pulses so the duration of the pulse is the time it take for the charge to go from source to ground. in addition the voltage is dependent on the resistance to group so the higher the resistance the greater the voltage and greater the time of the pulse. these pulse are from a PMT.
Are you using these pulses directly into a discriminator for NIM (nuclear instrumentation module) logic. or going into a pulse height analyzer? I have used photomultiplier tubes (10 stage and 14 stage) a lot, and usually get very fast (~10 ns) risetimes and ~30 to 40 ns fall times into 50 ohms for fast events. Nai(Tl) (sodium iodide) is very slow. We rarely needed any amplifiers. I cannot remember whether anode or last dynode signals are faster.
Bob S
 

FAQ: Bipolar transistors and pulse amplification

1. What is a bipolar transistor?

A bipolar transistor is a type of electronic device that uses two layers of a semiconductor material to control the flow of electric current. It consists of a collector, a base, and an emitter, and can be either a PNP or NPN type. It is commonly used in amplifiers, switches, and digital logic circuits.

2. How does a bipolar transistor work?

A bipolar transistor works by using a small current at the base to control a larger current between the collector and emitter. When a positive voltage is applied to the base, it allows current to flow from the collector to the emitter. When a negative voltage is applied, it blocks the flow of current. This allows the transistor to act as an amplifier or switch.

3. What is pulse amplification?

Pulse amplification is the process of increasing the strength or amplitude of an electrical signal in the form of pulses. This is commonly done using bipolar transistors, which can amplify small pulses of current into larger ones. This is important in applications such as audio amplifiers and digital circuits.

4. How is pulse amplification achieved using bipolar transistors?

Pulse amplification using bipolar transistors is achieved by connecting the transistor in a specific configuration called a "common-emitter" circuit. In this circuit, the input signal is applied to the base of the transistor, and the amplified output is taken from the collector. The amount of amplification can be controlled by the amount of current flowing into the base.

5. What are the advantages of using bipolar transistors for pulse amplification?

The main advantages of using bipolar transistors for pulse amplification are their high gain (amplification) and fast switching speeds. This allows for efficient and accurate amplification of pulse signals. Additionally, bipolar transistors are relatively inexpensive and widely available, making them a popular choice for various electronic applications.

Similar threads

Replies
11
Views
2K
Replies
3
Views
2K
Replies
37
Views
931
Replies
27
Views
2K
Replies
7
Views
4K
Replies
11
Views
8K
Back
Top