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I was watching how to make a nintoaster, that is, an NES (Nintendo Entertainment System) in the form of a toaster. The video is here. One of the things he does in the video is building a video-amp circuit for the video signals from the NES. The circuit looks like this (13:40 in the video).

enter image description here

I believe I understand somewhat what this circuit does (but feel free to point out if I'm wrong). The "VIDEO IN" signal controls the state of the transistor, that is how much current is flowing through it. The \$33\Omega\$ resistor is there to limit the current, and the "VIDEO OUT" is the output signal.

But I'm not sure why there is a \$220\Omega \$ connected to ground. What is the purpose of that resistor, if it even has any?

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But I'm not sure why there is a 220Ω connected to ground. What is the purpose of that resistor, if it even has any?

For any BJT amplifier you have to have a standing (or quiescent) DC current that passes from the positive supply (in the case of an NPN BJT), through the collector to the emitter and then to ground; in your case through the 33 Ω and 220 Ω resistors. It won't work without that standing BJT current. That current biases the transistor into correct operation.

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It establishes the output impedance. If you have this circuit driving a high impedance, you couldn't get much current flow in the circuit and the amplification would be dependent on the load impedance. This resistor puts the transistor in a current range that allows the circuit to work independent of the load impedance (within reasonable limits).

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The transistor pulls the output in the positive direction, the resistor in the negative direction; it’s simpler than having an amplifier that’s active in both directions and works very well if power is low and so efficiency isn’t too important.

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