This video is about the Gilbert cell which produces an output signal proportional to the product of two input signals. Such circuits are widely used for frequency conversion in radio systems. The advantage of this circuit is the output current is an accurate multiplication of the (differential) base currents of both inputs. As a mixer, its balanced operation cancels out many unwanted mixing products, resulting in a "cleaner" output.
Differential Amplifier: https://youtu.be/bongv60GqIY?si=nNVH23_1i4Yo_zeD
VCO https://youtu.be/4yC7b6otcJ4
CMOS Schmitt Trigger https://youtu.be/0WSm7NBRnao
uA741 Operational Amplifier: https://youtu.be/A0wn_SrZzeo