# Linear Voltage Regulators

• Sometimes called voltage references because they have low power capacity, and are often used purely as references in analog circuits. You probably wouldn’t be powering a motor off of these supplies.
• Linear supplies are assembled from passive (as opposed to actively controlled) components, and therefore don’t use switching or filtering as we will discuss later.
• All of these regulators function differently, the commonality between them is that they don’t use switching.

## Voltage Divider

• Good Voltage Divider Summary
• Can be purely built from resistors (or capacitors although less common).
• Probably what we are all most familiar with when we think about converting between DC voltages. Unfortunately poor efficiency - a good first step, but we can do a lot better than this.
• Output voltage is dependent on supply voltage + temperature and component ratings, as well as the impedance at our load.
• Imagine if R_1 = R_2 = 1000 ohms in the photo below. What would be V_o? What current would be flowing across R1 and R2 relative to our load, and would it be useful current? To get more current through our load what would we need to do to R_1 and R_2? The equations below are an oversimplification because our load is an open circuit.

## Zener Reference

• Good Zener Reference summary
• Before we discussed how diodes maintain a constant forward voltage drop
• Zener diodes can be manufactured with very specific reverse voltages (called their zener voltage).
• Unlike a voltage divider, zener references are tolerant to changes in their supply voltage, but still can change with thermals.
• Very commonly used due to accuracy and low part count

## Linear Power Amplifier

• Here’s a good TI whitepaper on LDOs, which are a type of linear power amplifier.
• Now let’s combine the voltage divider and zener reference with an op-amp and transistor to make a much more efficient and capable power supply. Think about what’s a reference, and what’s seeing the full voltage and current of our circuit (really just the transistor).
• We didn’t talk about op-amps in the components section, but they can be used to make more efficient voltage references without resorting to a switching supply.
• A linear power amplifier leverages the closed loop nature of an op amp, with a transistor, and therefore can supply greater current than an op amp output alone. *Note that you are driving the transistor in an intermediate state between cutoff and saturation where efficiency is poor!
• LDO’s which are commonly used to produce various voltage rails in microcontrollers and other ICs are regulators. Inductorless and passive - can be made entirely in silicon!
• In the equation below: V_L = V_Z * (1 + R_2 / R_1).
• If we know that the op amp will try to maintain a constant voltage across inverting and non-inverting inputs, can you see how its output (ties to the gate of the transistor) might tie back to provide feedback to ensure that is the case? Is the transistor fully of or on?