This is a continuation of a running series on the kinds of simple circuits we can do with basically just two transistors. In Part 1 we talked about Current source and Multi-vibrator, and in Part 2 we discussed Linear Regulators. We will continue with more practical circuits that can be very useful in many applications.
There are instances that we would like to translate a digital logic voltage from one level to another, say from 5V to 3V, or vice-versa. The input perhaps coming from a circuit that has a higher logic level that needs to input to a different logic level. For a single direction of signal, a single transistor would be enough. Bidirectional functionality can be achieved using two transistors, Bidirectional Logic Translator, as shown below.
R1 serves as a pull-up resistor to 5V in the circuit. The input typically comes from an open-drain or open-collector output, pulled up to supply by R1. The enable pin should be tied to the lower of the two supplies. When input is high, both Q1 and Q2 are off and the output is pulled to 3.3V. When the input is low or close to ground, Q1 turns on and the output is pulled close to ground. The 2nd transistor Q2 becomes useful here for opposite signal direction.
When enable pin is grounded, the output is completely isolated from the input.
Another circuit we would like to show is Two-Transistor Amplifier. Transistors are invented primarily to be amplifiers, though we find a lot of good use for them as switches. We can make an amplifier by simply using two transistors. Not as good as we would get from using operational amplifiers, but the circuit below does a good job of amplifying a signal by a gain of 2.
The gain is set approximately by R1/R5, with R2 = R6. Using transistors to make amplifiers requires careful biasing so that Q1 and Q2 operates in active region, and that there's enough headroom for signals. We'll have more circuits using two transistors in at least two more coming installments I intend to publish. Happy start of the week to all.
There are instances that we would like to translate a digital logic voltage from one level to another, say from 5V to 3V, or vice-versa. The input perhaps coming from a circuit that has a higher logic level that needs to input to a different logic level. For a single direction of signal, a single transistor would be enough. Bidirectional functionality can be achieved using two transistors, Bidirectional Logic Translator, as shown below.
R1 serves as a pull-up resistor to 5V in the circuit. The input typically comes from an open-drain or open-collector output, pulled up to supply by R1. The enable pin should be tied to the lower of the two supplies. When input is high, both Q1 and Q2 are off and the output is pulled to 3.3V. When the input is low or close to ground, Q1 turns on and the output is pulled close to ground. The 2nd transistor Q2 becomes useful here for opposite signal direction.
When enable pin is grounded, the output is completely isolated from the input.
Another circuit we would like to show is Two-Transistor Amplifier. Transistors are invented primarily to be amplifiers, though we find a lot of good use for them as switches. We can make an amplifier by simply using two transistors. Not as good as we would get from using operational amplifiers, but the circuit below does a good job of amplifying a signal by a gain of 2.
The gain is set approximately by R1/R5, with R2 = R6. Using transistors to make amplifiers requires careful biasing so that Q1 and Q2 operates in active region, and that there's enough headroom for signals. We'll have more circuits using two transistors in at least two more coming installments I intend to publish. Happy start of the week to all.
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