Transistor as a basic amp

In summary, the conversation discusses the confusion and difficulties encountered when trying to use transistors to amplify signals to a desired value. It is mentioned that the hfe value varies significantly and it is not reliable to depend on a specific value. The concept of using an emitter resistor and current mirror for voltage and current gain is also introduced. The importance of using feedback in amplifying devices is emphasized.
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From what I've read about transistors, I thought that I could find the hfe value in the data sheet, then apply a current to base and have the emmitter-collector throughput at a level of base*hfe. It sounded simple, and I was going to have a few different resistors for controlling base current, selected by a switch.

Well, I was confused off the bat, when looking at the data sheet. There was no 'nominal hfe'. There was a max hfe, a minimum hfe, and they were very different values. So how can I do what I wanted to do... know the output current via the base current? It sounds like I'm missing a general principle.

Thanks
 
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  • #2
hfe (or beta) varies wildly. The manufacturer will typically guarantee a minimum hfe but it is VERY bad design style to make a circuit that depends on a particular value of hfe.

Also, while physically the BJT is driven by base current, in practice it is more useful to think about it as a voltage amplifier with Ic exponentially related to Vbe via the diode equation.

Your idea for biasing the transistors would be almost impossible to make work because your transistors would slam themselves into saturation. Read this informative link for information on the proper way to bias a discrete BJT.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npncebias.html
 
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  • #3
Thank you. So, we cannot reliably use a transistor to amplify a signal to a desired value? It's just always an on/off switch in operation?
 
  • #4
You can use a transistor to reliably amplify a value. One way is to use an emitter resistor. Assume for a moment that Vbe is constant and an NPN transistor. You have 10 ohms in the emitter( to ground) and 1000 ohms in the collector (to Vcc). If you apply a signal to the base it will appear across the 10 ohm resistor causing a current to flow. This current will mostly come through the 1000 ohm resistor so there will be 100 times the voltage drop across the 1000 ohm resistor as across the 10 ohm resistor. There you have inversion and voltage gain. This is extremely oversimplified.

If you want reliable current gain there are ways to do that also.
 
  • #5
oneamp said:
Thank you. So, we cannot reliably use a transistor to amplify a signal to a desired value? It's just always an on/off switch in operation?

Of course you can use a transistor to reliably use a transistor to amplify a signal. It's how I make my living. But you can't do it in the way you described.

MeBigGuy is right and gave a simple way to get voltage gain. The gain in the structure he suggested is the ratio between the collector or emitter resistors.

If you need current gain, you should use a current mirror. Look that up in wikipedia. If you need high current gain, you should look at a transimpedance amplifier using a op amp.
 
  • #6
Thanks
 
  • #7
The point about using any amplifying device (linear and not just a switch) is that you should always use it with feedback, defined by the surrounding components. You end up with a well behaved amplifier at the expense of some loss of gain. But, hey, your average transistor, these days, has loads of gain to spare. It's not like in the old days when they had an Hfe of 10! (and I don't mean Factorial :wink:)
 
  • #8
sophiecentaur said:
The point about using any amplifying device (linear and not just a switch) is that you should always use it with feedback, defined by the surrounding components. You end up with a well behaved amplifier at the expense of some loss of gain. But, hey, your average transistor, these days, has loads of gain to spare. It's not like in the old days when they had an Hfe of 10! (and I don't mean Factorial :wink:)

Some day I'm going to really learn about transistors.
Today is not that day.
 

1. How does a transistor function as a basic amplifier?

A transistor is a semiconductor device that can control the flow of current between two terminals. In an amplifier circuit, the transistor acts as a switch, turning the input signal into a larger output signal. This is accomplished by using the transistor's ability to amplify the input signal through a process called gain.

2. What are the different types of transistor amplifiers?

There are three main types of transistor amplifiers: common emitter, common collector, and common base. In a common emitter amplifier, the input signal is applied to the base terminal and the output is taken from the collector terminal. In a common collector amplifier, the input is applied to the base terminal and the output is taken from the emitter terminal. In a common base amplifier, the input is applied to the emitter terminal and the output is taken from the collector terminal.

3. How do you calculate the gain of a transistor amplifier?

The gain of a transistor amplifier can be calculated by dividing the output voltage by the input voltage. This is known as the voltage gain and is typically denoted by the symbol "Av." The gain can also be calculated by dividing the output current by the input current, known as current gain or "Ai." The gain of a transistor amplifier is dependent on factors such as the type of amplifier circuit, the biasing of the transistor, and the properties of the transistor itself.

4. What is the purpose of using a transistor as an amplifier?

The main purpose of using a transistor as an amplifier is to increase the strength or power of a weak input signal. This is useful in various electronic devices such as speakers, radios, and televisions, where a small input signal needs to be amplified to produce a larger output signal. Transistor amplifiers are also commonly used in communication systems, audio equipment, and industrial control systems.

5. What are the advantages of using a transistor as an amplifier over other types of amplifiers?

Transistor amplifiers have several advantages over other types of amplifiers. They are small in size, making them suitable for use in compact electronic devices. They also have low power consumption and can operate at high frequencies. Additionally, transistor amplifiers have a high input impedance, allowing them to receive weak input signals without significantly affecting the input circuit. They are also relatively inexpensive to produce, making them a cost-effective option for amplification purposes.

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