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What Is a Transistor Amplifier?

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  • Written By: Solomon Lander
  • Edited By: A. Joseph
  • Last Modified Date: 02 September 2016
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A transistor amplifier is an electronic circuit that uses a semi-conducting transistor instead of a tube or integrated circuit chip to amplify electrical signals. Typically used in audio applications, a transistor amplifier provides excellent performance in a relatively small package. It has largely replaced the vacuum tube signal amplifier and remains a strong competitor to the more modern integrated circuit (IC) amplifier.

Before the invention of the transistor in 1947, amplifiers used vacuum tubes. Vacuum tubes were large, bulky, fragile and inefficient, and they required time to warm up. Transistors eliminated all of these problems while also offering the ability to amplify signals with much less distortion. In addition, they were able to output more powerful signals, allowing some transistor amplifiers to output hundreds of watts per channel. Their small size and low power consumption also made possible the invention of battery-powered portable audio components, such as transistor radios.

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The structure of a transistor amplifier circuit is relatively simple. In it, a power supply is connected to the transistor’s collector terminal, and the signal to be amplified gets fed into the base terminal. The transistor uses the signal at the base to determine how much power from the collector flows through its gate to the emitter terminal, which transfers the amplified signal. If a transistor is compared to a faucet valve, the collector would be the supply pipe, the emitter would be where the water comes out, and the base would be the hand that turns the spigot on, off or somewhere in between.

Amplifiers using IC chips began to replace the transistor amplifier in the 1960s. The IC chip combined multiple electronic components onto one small piece of silicon, allowing it to do more in much less space. Bad sound quality and very limited power output capabilities plagued these types of amplifiers. Over the years, though, the technology has improved to the point that most portable and lower-cost home audio components use IC amplifiers.

Even with low-cost ICs, many home audio components still use transistor amplifiers, although they are frequently described as discrete amplifiers. This circuit type is more prevalent in power amplifiers and in the final output stage of amplifiers, both of which take the line-level signal from the preamplifier and amplify it for output to speakers. Some high-end source components and preamplifiers also use transistor amplifiers, though. In either case, these amplifier circuits use metal oxide semiconductor field-effect transistors (MOSFETs) as the source of amplification.

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sunshined
Post 4

@myharley - My mom still uses a transistor radio on a regular basis. She spends a lot of time outside gardening, and likes to listen to her talk shows on the radio.

She barely uses a cell phone, so having something like an Ipod is nothing she would even consider. She loves her transistor radio though.

She has this small radio on a rope that hangs around her neck so she can listen as she works. If there aren't any good talk shows on, she will listen to music.

I really don't think it is a bad idea to keep something like this around in case of emergencies. They are really cheap, and you can still easily find them available.

myharley
Post 3

I know I am dating myself here, but I remember transistor radios. My kids have no idea what I am even talking about if I mention one.

This was a small black radio with an antenna and knobs you could roll back and forth to tune in to a station.

Because these ran on batteries, you knew you could always listen to the weather or any other information if you ever lost power.

This little radio got used quite a bit, so there were usually always working batteries in it. I can still here the crackling sound when you were turning to dial to find a station.

I haven't seen one for years. I wonder if they still make transistor radios?

miriam98
Post 2

@hamje32 - I was surprised to read that the early IC chips weren’t as good as the transistor amplifiers. I would have thought that they would have been much better.

Most IC chips, after all, consist of many transistors put on the chip. I guess they had not refined the technology when the chips first came out so the transistor amplifiers were still the better technology.

hamje32
Post 1

I had one of those old vacuum tube amplifiers that the article talks about. No, I am not that old, but it was a vintage heirloom handed down in the family.

That thing still worked. It would take awhile to power up, and squeal when it was turned on. It ran very hot too, taking awhile to cool down when it powered down.

It did a good job of powering up the ancient audio theater system that went with it too. Of course that stuff is antique now. Years later I went to Radio Shack and bought my first theater system using transistor amplifiers and I thought the newer system was pretty sweet.

It didn’t have the sentimental value that the antique did, but it was compact and delivered vibrant sound. I still have the antique, however. I may sell it on Ebay one day.

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