Using a transistor to amplify current

In summary, you can't use the base emitter voltage to calculate collector current because the current is limited by the base current. You can use a resistor to limit the current.
  • #1
SMD1990
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Hello. As the title says, I need some help in getting a transistor to amplify a current.

I have a TIP120, whose DC gain is specified as 1000. I am applying a positive voltage of about 0.6 V to the base. The current is around 10 μA.

I have the positive side of a AA cell connected to the collector. The emittor connects to the negative side of the "battery", and to the negative of my 0.6 V signal being applied to the base.

Why am I not reading around 10 mA from the battery? Turning off the base voltage causes the reading to become zero. Still, when the signal is applied, I am only reading 20 to 30 μA.
 
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  • #2
SMD1990 said:
Hello. As the title says, I need some help in getting a transistor to amplify a current.

I have a TIP120, whose DC gain is specified as 1000. I am applying a positive voltage of about 0.6 V to the base. The current is around 10 μA.

I have the positive side of a AA cell connected to the collector. The emittor connects to the negative side of the "battery", and to the negative of my 0.6 V signal being applied to the base.

Why am I not reading around 10 mA from the battery? Turning off the base voltage causes the reading to become zero. Still, when the signal is applied, I am only reading 20 to 30 μA.
0.6V is only an approximation. Most likely the transistor is not even turn on. Usually it is about 0.7V. BUT BUT! You cannot hook up circuit like this, because the circuit is too sensitive that as soon as you raise the 0.6V towards 0.7V, the transistor will suddenly turn on and draw a lot of current and drain your battery or burn the transistor.

To play with this, you need to put a small resistor between the emitter and the negative of the battery. Say a 100ohm. So when you slowly increase the base voltage from 0.6V up, you'll see there is a voltage drop across the 100ohm emitter resistor. from that, you can calculate the collector current as the current gain you gave is 1000. The reading should be close.

this little 100ohm is called degeneration resistor that help to stabilizing the circuit.
 
  • #3
My multimeter's diode testing mode shows the drop between the base and emitter as 0.55 V.

The short-circuit current of the AA should be less than the transistor's maximum rating. Besides, should not the limited base current keep the collector current limited to well below the maximums?

If not, my reasoning is incorrect.
 
  • #4
SMD1990 said:
My multimeter's diode testing mode shows the drop between the base and emitter as 0.55 V.

The short-circuit current of the AA should be less than the transistor's maximum rating. Besides, should not the limited base current keep the collector current limited to well below the maximums?

If not, my reasoning is incorrect.

The base emitter voltage change with emitter current, you cannot use that as the number because your meter drive very little current. You did say the gain is 1000, so if you put 1mA into the base, it will give you 1A at the emitter, that would be enough to pull down and drain the battery quickly. Check out some practical transistor circuits to experiment.

There is a way to ground the emitter and drive the base alone without the emitter degenerating resistor, but people use a high value resistor in series with the base to drive. Say if you solder the emitter to the negative side of the battery and collector to the positive. Then you put a 1M resistor in series with the base and drive the resistor. Then you can look at the collector vs the voltage drop across the 1M resistor and back calculate the beta of the transistor.
 
  • #5
look up datasheet for TIP120

if Beta (hfe) is 1000 it's almost certainly a Darlington which will require 1.2 volts base-emitter because there's two junctions to overcome not one.


"Darlington" is a method of connecting two transistors piggybacked.
It's so handy the manufacturers sell them in one case, as shown on this Fairchild datasheet:
www.fairchildsemi.com/ds/TI/TIP120.pdf

observe from datasheet absolute maximum base current is 0.12 amp, and a AA cell might well exceed that.
if you've ever had the battery directly across base-emitter it likely shorted one of the junctions and that could be why you read low Vbe of .55 v --- transistor wrecked.

But maybe not - Try it again at Vce of 3 volts and it might work.

but monitor and limit base current.

Also observe pinout is BCE not EBC,,,, might your 0.55 volt be a hookup error?
 
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FAQ: Using a transistor to amplify current

What is a transistor and how does it work?

A transistor is a semiconductor device that is used to amplify or switch electronic signals. It consists of three layers of material, namely the emitter, base, and collector, which control the flow of current through the transistor. By applying a small current or voltage at the base, a larger current can be controlled between the emitter and collector, making it useful for amplifying signals.

What are the benefits of using a transistor to amplify current?

Using a transistor to amplify current has several benefits, including a higher gain (amplification) compared to other components, a smaller size, and lower power consumption. It also allows for precise control of the amplified signal, making it useful in a wide range of electronic devices.

How do you calculate the amplification factor of a transistor?

The amplification factor, also known as the gain, of a transistor can be calculated by dividing the output current by the input current. This is known as the DC current gain, and it can also be calculated by dividing the output voltage by the input voltage. Other factors such as temperature and biasing can also affect the amplification factor.

Can a transistor amplify alternating current (AC) signals?

Yes, a transistor can amplify both direct current (DC) and alternating current (AC) signals. However, it is important to properly bias the transistor and use appropriate coupling methods to ensure the fidelity of the amplified AC signal. Additionally, different types of transistors may have different frequency responses, so it is important to choose the right type for the desired application.

What are some common applications of using a transistor to amplify current?

Transistors are used in a wide range of electronic devices, including audio amplifiers, power supplies, radio and television receivers, and computer circuits. They are also used in sensors, switches, and other electronic components. In general, any application that requires amplification of electronic signals can benefit from using transistors.

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