- [Lecturer] Although BJTs and MOSFETs can be used
for similar purposes they are different types
of transistors that work in different ways
and each have their own set of pros and cons.
Deciding whether to use a BJT or a MOSFET
for certain projects can be a bit of a struggle
and unfortunately there isn't a straightforward answer
for which one is best to use,
it depends on what you're doing.
So, to help with planning your next electronics project
I'll discuss some of the pros and cons of each one.
BJTs tend to have better, more linear gain characteristics
and can give you a lot higher voltage gain than MOSFETs.
They're also able to handle higher output currents
and have a lower output impedance.
That gives BJTs a huge advantage over MOSFETs
for building amplifier circuits
that need to provide a significant amount of output power
and or drive loads that have low input impedance.
MOSFETs are going to have a harder time
driving a low impedance load
because they have a higher output impedance.
That said, the advantage of MOSFETs
is that they have a very high input impedance,
so it's easier to couple them with a signal source
that has a higher output impedance.
Trying to connect a BJT amplifier
to a high impedance signal source
can cause problems, because the BJT
has a lower input impedance.
Another advantage of MOSFETs is that
since they're voltage controlled
rather than current controlled
they usually consume less power than a BJT.
A MOSFET doesn't require any current to flow
through its gate terminal to operate,
but a BJT consumes base current whenever it's turned on.
That power efficiency can make MOSFETs
more desirable for battery-powered applications,
where every little bit of power that gets spent matters,
especially when the load that it's controlling
is variable and could have a low impedance.
However, for controlling a load that draws a fairly small
and predictable amount of current, like an LED,
using a BJT isn't all that bad.
You can choose the right limiting resistor
to keep the base current as small as possible
to achieve a reasonable efficiency.
When directly driving a transistor
with the 3.3 or five volt output from a microcontroller
it's usually easier to find BJTs
that can turn with those voltages than MOSFETs,
because MOSFETs usually require a gate-to-source voltage
of at least 10 volts to fully turn on.
That makes BJTs the better choice for driving
low-powered devices directly from microcontrollers
and BJTs will usually be cheaper
than the corresponding MOSFET that can do that same job.
For high power applications, driving loads
that require significant and variable amounts of current,
like a large motor, MOSFETs are generally the better choice.