While reading Jay Carlson's article on embedded Linux processors, I saw this schematic on the site:
I don't recognize the components in the Q1 and Q2 sections. I think it's some sort of transistor, but I'm not sure. Does anyone know what this is?
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\$\begingroup\$ It looks something like one of these \$\endgroup\$– Peter K.Jun 6 at 21:43
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\$\begingroup\$ These function just like an H-bridge in a switch mode power supply. Just replace the speaker with the primary of the transformer. Only with two complimentary transistors in a package, instead of 4 separate transistors. \$\endgroup\$– SteveShJun 10 at 23:17
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1 Answer
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Q1 and Q2 are dual, complementary (n-channel and p-channel), power MOSFETs; two transistors in one package.
If you call the two FETs in each symbol u and d (for up and down), then the circuit action is like this:
First, Q1u and Q2d are turned on. This creates a current path from Vdd through the speaker from top to bottom, to GND. You cut off the bottom of the schematic, but GND is a reasonable assumption.
Next, those two transistors are turned off, and Q1d and Q2u are turned on. This creates a current path from Vdd through the speaker from bottom to top, to GND.
The effect is that the speaker "sees" a peak-to-peak driving waveform of 2 x Vdd. The term for this is BTL - Bridge-Tied Load. It is a way to get more power into a speaker without increasing the available power supply voltage.
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\$\begingroup\$ It's also convinient as you don't need to have a split rail supply or use a large capacitor to AC couple the speaker. \$\endgroup\$ Jun 11 at 2:24