Common collector vs Common drain amplifier

In summary, the main advantage of using a FET over a BJT for a common drain amplifier is that the FET has a lower threshold voltage, making it better suited for low voltage applications. The main disadvantage of using a FET over a BJT for a common drain amplifier is that FETs are voltage controlled devices, whereas BJT's are current controlled devices, and FETs perform badly at low voltages.
  • #1
boredaxel
19
0
I was wondering what's the main advantage of choosing one over the other, besides common drain having to suffer body effect. How do we determine if a CC is better or a CD is better to use as a buffer?
 
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  • #2
You would use a common drain amplifier if you were using a FET. If you had a BJT transistor, you would use a common collector circuit.

Both have slightly less than unity gain (gain < 1 ) and low output impedance, so they are used as buffer amplifiers.
Common collector:
130px-NPN_emitter_follower.svg.png

Common drain:
180px-N-channel_JFET_source_follower.svg.png
 
  • #3
The main operational difference (at least in my mind) is that FET's are Voltage controlled devices, whereas BJT's are current controlled devices.
 
  • #4
With Vcc=5 volts, BJTs (common collector) work much better than NMOSs (common drain).
Bob S
 
  • #5
The threshold of a FET is several volts, but only 0.65V for a bjt. If the input is 10V, the FET will output around 7V, where as the bjt will output 9.35V. For low voltages, the FET threshold is a problem. As Bob. S, stated, when the voltage is LOW, the bjt is a better choice since it drops much less voltage.

Claude
 
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  • #6
The threshold of a FET is several volts,
You wouldn't use an enhancement mode FET as a common drain amplifier. They do have a large threshold voltage.
Something like an MPF102 actually requires a negative voltage on the gate (relative to the source) to bias it.
This makes the biassing a little more complex as a large source resistor may be needed for the load but a smaller one may be needed for the biassing. So, such circuits often have both.
They have a small bypassed source resistor for bias purposes and a larger unbypassed one for the load. A gate resistor takes the voltage between the two to the gate.

However FETs perform badly at low voltages as Bob S has pointed out.
The high input impedance of an emitter follower is surprising if you haven't seen it before and it certainly becomes a voltage driven device.

If the supply voltage is 10 volts or so, which one you use doesn't make much difference to the performance.
 
  • #7
vk6kro said:
[Something like an MPF102 actually requires a negative voltage on the gate (relative to the source) to bias it.
This makes the biassing a little more complex as a large source resistor may be needed for the load but a smaller one may be needed for the biassing. So, such circuits often have both.
They have a small bypassed source resistor for bias purposes and a larger unbypassed one for the load. A gate resistor takes the voltage between the two to the gate.
.
Finally getting back to the "good ol' days" of the familiar cathode bias circuits.

Bob S
 
  • #8
Thanks everyone for the input!
 

What is the difference between a common collector and common drain amplifier?

A common collector amplifier is a type of circuit where the input signal is applied to the base of a transistor and the output is taken from the emitter. In a common drain amplifier, the input signal is applied to the gate of a field-effect transistor (FET) and the output is taken from the source.

Which amplifier is more commonly used in electronic devices?

Common collector amplifiers are more commonly used in electronic devices because they have a higher voltage gain and lower input impedance compared to common drain amplifiers.

What is the purpose of using a common collector or common drain amplifier?

The purpose of using a common collector or common drain amplifier is to amplify a small input signal to a larger output signal. This is commonly used in audio amplifiers, signal processing circuits, and communication systems.

Which type of amplifier has a higher output impedance?

A common drain amplifier has a higher output impedance compared to a common collector amplifier. This means that it is better suited for driving high impedance loads, such as speakers or long transmission lines.

Can a common collector and common drain amplifier be used together in a circuit?

Yes, a common collector and common drain amplifier can be used together in a circuit to achieve a higher overall voltage gain. This is commonly known as a cascode amplifier, where the output of the common collector amplifier is connected to the input of the common drain amplifier.

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