I'm so curious, What is the different of putting load series or parallel with resistor like below circuit

enter image description here

the circuit will be used just for switching circuit with VCC higher than the input Is there any correlation with current flow through the load? which is better for the switching?

if i use the circuit for the amplifier (of course there will be a Voltage Divider Biasing circuit and Resistor and capacitor in emitter), which is better?

thank you


2 Answers 2


The first circuit

the transistor acts as a switch(Assuming enough base current to drive the transistor into saturation).

When the transistor is ON the motor also turn ON and almost all the supply voltage Is given to the motor(except for the Collector-Emitter drop which depends on Collector current and may be up to 1V for this transistor if the motor draws large Current).

So,except for the drop on the transistor almost all the power from the supply is Given to the motor.Which is interpreted into more efficiency.Specially for Battery powered applications power dissipation is of great concern.

The second circuit

When the transistor is Off it acts as an "Open" and the current finds no other Path except through the motor.

But with Vcc supplying current to the motor Through the series connected resistor "R3" which drops an amount of voltage That's directly related to the Amount of current the motor draws and its(R3) Resistance value.Which leads to significant voltage drop across R3 and hence large power dissipation.

Also note that R3 prevents the motor from working with its full power since R3 limits the current through it.

All the above are interpreted into very low efficiency and waste of power.

One thing that's important too.For the circuit to the left when the motor is OFF Neither the transistor nor the motor draws current,but for the circuit to the Right either the transistor or the motor draws current.

For amplifier operation the second circuit is common because the quiescent point And the voltage gain all depend on the collector resistor which in the Case of the first circuit will be the load resistor and for all practical Purposes the load is not precisely known.

But if you consider the second circuit.You can choose the value of the collector Resistor to give you the desired gain and Q-point and connect whatever load you Want to the collector of the transistor without affecting the amplifier Parameters significantly,As long as, the load is within the allowed range.


In the first circuit, the transistor is acting like a switch that either applies power to the motor or it doesn't. If you want to switch a motor on/off, that's a good way.

In the second circuit, the motor is powered thru the resistor. That by itself is a problem since the resistor will drop some voltage and dissipate some power. The motor won't see the full supply voltage.

Note that the motor is on (to the extent the resistor allows enough current) when the transistor is off. Turning on the transistor turns off the motor. However, this is done inefficiently. The motor is being turned off by the transistor robbing the current that would otherwise go to the motor. This means the resistor dissipates even more power when the motor is off.

When amplifying signals, as opposed to switching power, a circuit like the second can be useful.

  • \$\begingroup\$ @MCG: Thru is a alternate spelling for through that I prefer. Stop changing it. I've let a bunch of your edits go because you fixed other things too, but this time that's all you did. I've had enough, and rolled back your edit. \$\endgroup\$ Commented Apr 13, 2018 at 10:38
  • 3
    \$\begingroup\$ it is the non standard, and should be considered slang. Definitely should not be used in any sort of formal writing at all. It's really just laziness. \$\endgroup\$
    – MCG
    Commented Apr 13, 2018 at 10:47
  • 3
    \$\begingroup\$ It doesn't change the fact that it is non standard, and should definitely not be used in any sort of formal way. I know it's deliberate as you have the same error repeatedly. Unless u wnt 2 strt speeking lik dis frm now on? Because lazy writing is all it is good for. \$\endgroup\$
    – MCG
    Commented Apr 13, 2018 at 10:57
  • 1
    \$\begingroup\$ Thru: North American, informal, non standard spelling of the word 'through'. Same as spelling 'you' as 'u'. Essentially, it is lazy and incorrect, so I believe it is valid to correct it \$\endgroup\$
    – MCG
    Commented Apr 13, 2018 at 13:28
  • 3
    \$\begingroup\$ So what you are saying is because you prefer to do something incorrectly, it means it shouldn't be corrected? As you are someone who is often rude to people about spelling and grammar of their questions or answers, I would have thought you would want your English to be correct. No need for the whining. \$\endgroup\$
    – MCG
    Commented Apr 16, 2018 at 12:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.