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bjt vs mosfet in digital applications

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_Mark81_

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Hello,
let's talk about the control of digital lines. I'm thinking about small loads (leds, buzzers, ...) or bus management (ps2 host-to-device and so on).

One might use either a small bjt (i.e. BC817) or a small logic-level mos (i.e. 2N7002K or BS170).

What are the advantages and disadvantages of both ways?

Thanks in advance
Mark
 

in discrete design BJT has many advantages such as speed.
 

ahmad1954 said:
in discrete design BJT has many advantages such as speed.
Click to expand...
Actually MOSFETs are usually faster than BJTs in switching applications because they don't suffer from storage delay.

Also MOSFETs don't require any DC gate current so they are more efficient for low frequency switching since a BJT requires about 10% of the collector current to drive the base.

At higher frequencies it evens out since you need to supply the current to charge and discharge the relatively large gate capacitance of a MOSFET. This average gate current increases with switching frequency and may exceed that required by a BJT at high frequencies.
 

I agree, but what about applications where there are no requirements about fast switching or power consumption?
I mean, what about the reliability in automotive application for example?
 

_Mark81_ said:
I agree, but what about applications where there are no requirements about fast switching or power consumption?
I mean, what about the reliability in automotive application for example?
Click to expand...
I would think they are both basically have the same reliability in such an application as long as they are operated well within their voltage, power, and current ratings.
 

BJTs at voltages over about 5V BVceo will be faster than
a MOSFET of equal BVdss, provided you stay out of
saturation.

When I first started designing ICs, early '80s, I was doing
single (bipolar) transistor logic, Schottky clamped, and it
ran rings around the CMOS of the day (~ 1.5um, 5V).

Even today a SiGe HBT that is matched to its CMOS host
is much faster than that CMOS.

Reliability comes down tradeoffs, protection and abnormal
conditions most likely, and that comes down to cases. If
automotive is your application, chase parts with that
keyword (the automotive electrical environment is nasty
and some serious derating is in order, along with, perhaps,
some repetitive avalanche ruggedness).
 

dick_freebird said:
BJTs at voltages over about 5V BVceo will be faster than
a MOSFET of equal BVdss, provided you stay out of
saturation.

..............
Click to expand...
That may be true but unsaturated bipolar digital circuits (such as emitter-coupled logic) seem rather outside the realm of the op's question about controlling small switched loads (LEDs, buzzers,..) or PS2 bus management, which are likely never done with unsaturated BJT circuits.
 

If you use a N-channel BJT as a switch, if you apply current to the base that means it's "on" and when that happens yo have a voltage drop of 0.7V between base-emitter nodes, take that 0.7V and multiply it times the base current and you get the the power the transistor itself is consuming when it's "on". Unlike the BJT the CMOS transistor does not conduct current between the gate (base) and the drain (emitter) because they are isolated by a dielectric (much like a capacitor), thus not consuming any power while in a steady state (either on or off). So there's the answer, a BJT consumes power while "on" and the CMOS doesn't, the CMOS consumes power just in the instant that switches from on to off and viceversa, so they consume more power in high frequency (HF) applications.
 
_Mark81_ said:
I agree, but what about applications where there are no requirements about fast switching or power consumption?
I mean, what about the reliability in automotive application for example?
Click to expand...

an_82 said:
If you use a N-channel BJT as a switch, if you apply current to the base that means it's "on" and when that happens yo have a voltage drop of 0.7V between base-emitter nodes, take that 0.7V and multiply it times the base current and you get the the power the transistor itself is consuming when it's "on". Unlike the BJT the CMOS transistor does not conduct current between the gate (base) and the drain (emitter) because they are isolated by a dielectric (much like a capacitor), thus not consuming any power while in a steady state (either on or off). So there's the answer, a BJT consumes power while "on" and the CMOS doesn't, the CMOS consumes power just in the instant that switches from on to off and viceversa, so they consume more power in high frequency (HF) applications.
Click to expand...

+1
Very good explanation
 
an_82 said:
If you use a N-channel BJT as a switch, if you apply current to the base that means it's "on" and when that happens yo have a voltage drop of 0.7V between base-emitter nodes, take that 0.7V and multiply it times the base current and you get the the power the transistor itself is consuming when it's "on". Unlike the BJT the CMOS transistor does not conduct current between the gate (base) and the drain (emitter) because they are isolated by a dielectric (much like a capacitor), thus not consuming any power while in a steady state (either on or off). So there's the answer, a BJT consumes power while "on" and the CMOS doesn't, the CMOS consumes power just in the instant that switches from on to off and viceversa, so they consume more power in high frequency (HF) applications.
Click to expand...
A couple of minor nits on definitions. :wink:

BJT's are normally referred to as N-type or P-type. N-channel or P-channel refers to MOSFETs.

CMOS is Complementary MOS logic which uses both N-channel and P-channel transistors. If you are referring to a single transistor then it's a MOSFET.
 

crutschow said:
A couple of minor nits on definitions. :wink:

BJT's are normally referred to as N-type or P-type. N-channel or P-channel refers to MOSFETs.

CMOS is Complementary MOS logic which uses both N-channel and P-channel transistors. If you are referring to a single transistor then it's a MOSFET.
Click to expand...

Dear crutschow,
you are right! N-type BJT, N-channel MOS
 

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