Transistor various frequencies

In summary, transistors are able to output various frequencies at once by using negative feedback to eliminate distortions and variations in characteristics. They do this by producing a single complex waveform that represents the sum of all the individual components. The output transistor never actually sees multiple waveforms as they are merged into one when the original sound was produced. The current takes care of itself thanks to Ohm's law. The overlap of frequencies can cause issues in audio reproduction, which is why multiple speakers are often used to cover different frequency ranges.
  • #1
Salvador
505
70
Good day ,

Transistor characteristics tend to change under load and at various frequencies and teperatures etc, but when we have a simple linear class ab amplifier with two bjt output devices , I wonder how does a single transistor output various frequencies at once? I understand it's controlled by the input signal which is amplified itself and then drives the base of the output transistor but still.

Music is various frequencies which differ also in the loudness which translates to voltage/current , so how does the transistor manage to output more than one voltage/current waveform at once or simultaneously?
 
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  • #2
Clearly it can't output "more than one voltage" because there is only one output pin. It just outputs one very complex waveform that represents the sum of all the individual components.
 
  • #3
Salvador said:
Good day ,

Transistor characteristics tend to change under load and at various frequencies and teperatures etc, but when we have a simple linear class ab amplifier with two bjt output devices , I wonder how does a single transistor output various frequencies at once? I understand it's controlled by the input signal which is amplified itself and then drives the base of the output transistor but still.

Music is various frequencies which differ also in the loudness which translates to voltage/current , so how does the transistor manage to output more than one voltage/current waveform at once or simultaneously?
Transistors are inherently very non linear but that really doesn't matter.
The whole point about 'modern' (i.e. since they started building them) amplifier design is that negative feedback is used to eliminate distortions and variations in characteristics from device to device. You start with enough gain to blow your hat off and then you use feedback to make the output voltage (or current or whatever) as near a match to the input signal as you need. Transistors are inherently very non linear but that really doesn't matter.
 
  • #4
It just outputs one very complex waveform that represents the sum of all the individual components.

Hmm, I;m not sure I understood , I did understand however that the single output pin of a transistor can't have many frequencies and voltages/currents at once but when there is a simple record of say a heavy bass line and a drum dish being hit at the same instant , the high pitch is a low voltage high frequency sound while the bassline is a low frequency much bigger voltage and current demanding sound for the speaker , so how does the transistor output these two different ones at once? does the little high pitch waveform is " put on" the lower longer wavelenght sound ?Like smaller waves on top of a big wave in sea?
 
  • #5
Salvador said:
Hmm, I;m not sure I understood , I did understand however that the single output pin of a transistor can't have many frequencies and voltages/currents at once but when there is a simple record of say a heavy bass line and a drum dish being hit at the same instant , the high pitch is a low voltage high frequency sound while the bassline is a low frequency much bigger voltage and current demanding sound for the speaker , so how does the transistor output these two different ones at once? does the little high pitch waveform is " put on" the lower longer wavelenght sound ?Like smaller waves on top of a big wave in sea?

There can be only one value of voltage at anyone time. The value will vary over time and this is the 'time domain' description of the signal. You can also describe the signal in terms of a number of frequency components (frequency domain). These all vary at different frequencies and, at anyone particular instant, they add up to give the single voltage value.

You should think in terms of one domain at a time - at least whilst you are learning about this stuff or you can fall over yourself. True Fourier Analysis operates over an infinite time and gives you a continuous 'spectrum'. You can, however, do almost as well with the Discrete Fourier Transform which looks at a finite length of a signal. This will describe the signal as a series of discrete harmonics.
 
  • #6
Salvador said:
Hmm, I;m not sure I understood , I did understand however that the single output pin of a transistor can't have many frequencies and voltages/currents at once but when there is a simple record of say a heavy bass line and a drum dish being hit at the same instant , the high pitch is a low voltage high frequency sound while the bassline is a low frequency much bigger voltage and current demanding sound for the speaker , so how does the transistor output these two different ones at once? does the little high pitch waveform is " put on" the lower longer wavelenght sound ?Like smaller waves on top of a big wave in sea?

Exactly.

Plot the two voltage waveforms vs time.
Add up the voltages.

The current takes care of itself thanks to ohms law (speaker modeled as a constant resistance).

PS In case not obvious..The output transistor never actually sees two separate waveforms as you describe. They were merged into one complex waveform when the original sound was produced/recorded.
 
  • #7
Another way to convince yourself is to think that the loudspeaker cone can only be in one place at once and it is 'carrying' all those sounds.
 
  • #8
Oops duplicate post deleted
 
  • #9
Another way to convince yourself is to think that the loudspeaker cone can only be in one place at once and it is 'carrying' all those sounds.


Guess this is also the reason for high range , mid range and low range speakers and dividing in frequencies for better audio reproduction.
Seems like the average speaker suffers more from frequency overlap than the transistors because a speaker is a physically moving device ?
 
  • #10
I am not sure what you mean by the "frequency overlap". The only reason for using more than one mechanical driver unit is because of the large number of octaves involved in audio and the difficulty of matching a single drive unit to the air impedance. It can be done with impracticably large horn loudspeakers, though. The distortions involved with multiple crossover filters introduces additional problems and it's all a matter of compromise.
 

1. What is a transistor and how does it work?

A transistor is a semiconductor device that can amplify or switch electronic signals. It consists of three layers of material, each with a different level of doping (adding impurities to change its electrical properties). By applying a voltage to the middle layer, known as the "base", the transistor can control the flow of current between the other two layers, known as the "emitter" and "collector". This makes it an essential component in modern electronics.

2. What is frequency and how does it relate to transistors?

Frequency refers to the number of cycles a signal completes in one second, measured in hertz (Hz). Transistors are often used in circuits that involve alternating current (AC), which means the current is constantly changing direction. The frequency of this alternating current determines how often the transistors need to switch on and off, and this can have an impact on the performance of the circuit.

3. How do transistors handle various frequencies?

Transistors can handle a wide range of frequencies, from a few hertz to many gigahertz (GHz). The frequency handling capability of a transistor depends on its design and construction. Higher frequency transistors have shorter base regions and thinner layers, allowing them to switch on and off faster. Different types of transistors are designed for specific frequency ranges, such as radio frequency (RF) transistors for high frequency applications.

4. What factors can affect the frequency response of a transistor?

There are several factors that can affect the frequency response of a transistor, including its physical construction, the materials used, and the voltage and current levels applied. The parasitic capacitance and inductance of a transistor can also have an impact on its frequency response. Additionally, temperature changes and external noise can affect the performance of a transistor at different frequencies.

5. How do engineers select the right transistor for a specific frequency range?

Engineers consider several factors when selecting a transistor for a specific frequency range. This includes the required frequency range of the circuit, the impedance of the circuit, and the power requirements. They also consider the transistor's gain and noise characteristics, as well as its stability and reliability. Additionally, engineers may use computer simulations to analyze and compare the performance of different transistors before making a final selection.

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