I wanted to build a simple amplifier for a square wave generator with the following configuration: enter image description here

The presence of the amplifier is crucial since with my 12Ω load the output peak current may be near to 2A.

My main problem with this configuration is that my square wave generator is (and must be, for my project) adjustable. So, I can choose its amplitude value and set it from 0 to 20V, while the voltage supply is 24V.

Since the voltage supply is 24V, there will be a huge voltage drop between collector and emitter terminals, which will become heat.

Do you have an alternative configuration which avoids this problem?

It's exactly the same problem of a general linear voltage regulator. In such a case it's solved by choosing a PWM switched mode regulator. But that's true for a DC voltage regulator. Is there a similar configuration for a square wave voltage regulator?

  • \$\begingroup\$ Is the load purely resistive? Does the frequency vary? \$\endgroup\$ Apr 5, 2021 at 8:37
  • \$\begingroup\$ Resistive Load, constant frequency \$\endgroup\$
    – Kinka-Byo
    Apr 5, 2021 at 8:39
  • \$\begingroup\$ "Do you have an alternative configuration which avoids this problem?" Yes, PWM. In PWM the transistor is either fully off so \$ P = VI = V\times 0 = 0 \$ or fully on so in your case \$ P = 0.2 \times 2 = 0.4 \ \text W \$. (0.2 V transistor saturation voltage and 2 A). That's why it's used so much. With your scheme at 12 V out you'll have \$ P = VI = 12 \times 1 = 12 \ \text W \$. (All voltages in the calculations are across the transistor.) \$\endgroup\$
    – Transistor
    Apr 5, 2021 at 8:53
  • \$\begingroup\$ @Transistor Can you tell me how can I use PWM for a square wave regulation? \$\endgroup\$
    – Kinka-Byo
    Apr 5, 2021 at 11:26
  • \$\begingroup\$ Maybe, if you can tell us what it is you are making and what the 12 ohm load represents. Edit all the details into the question. \$\endgroup\$
    – Transistor
    Apr 5, 2021 at 11:38

1 Answer 1


For a high-side switch, when your input signal level is smaller than the load voltage, you generally need two transistor stages.

The resistor values and transistor types are approximate, you need to analyze for your requirements.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Won't the voltage drop on Q1 collector-emitter terminals cause heat like in my initial configuration? \$\endgroup\$
    – Kinka-Byo
    Apr 5, 2021 at 8:59
  • \$\begingroup\$ Well you do need to properly select the transistors, yes, but there's low effective on resistance transistors - though you might have to replace Q2 with a MOSFET if a BJT can't get low enough. But quite likely it can. \$\endgroup\$
    – mmmm
    Apr 5, 2021 at 9:27
  • \$\begingroup\$ I think I misunderstood the question. If you want a variable square wave, I would use a circuit similar to this, but put a buck regulator before it. If you require high precision on your square wave voltage, you will need to do it differently. \$\endgroup\$
    – Mattman944
    Apr 5, 2021 at 13:24

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