Voltage regulator
Goals:
Design a voltage regulator for positive voltages
Understand efficiency and regulation of a regulator
Appreciate some fine points of small signal “ground”
Understand how to compensate errors due to opamp input bias currents
Specifications:
A voltage regulator is simply a feedback driven voltage controlled voltage source shown above. The input is a temperature stable dc reference(from a bandgap reference, a Zener diode, or a string of forward biased diodes). The feedback resistor can be made variable to have a variable supply such as the one on your bench. In your implementation, realize output voltages of 3V and 6V by shorting out a part of R1 as shown above.
In your case, the “opamp” will be realized using bipolar transistors. Therefore, transistors in the input differential pair draw a bias current IB. Determine the output voltage in presence of bias currents. How would you overcome this error?
The above figure shows the complete schematic of the opamp. Cc is a compensation capacitor. In the small signal picture, Cc should be conencted between the base of Q3 and ground. Where should it be connected in the large signal picture? (Hint: think about what happens to the output voltage if there is a jump in the power supply voltage).
Wire up the amplifier and apply a small signal square wave riding around 1.2V at the reference input and RL=∞. Does the amplifier settle without ringing? If not, compensate the loop using a capacitor Cc as shown in the figure-connect it to the appropriate small signal ground determined above. Start from small values of Cc and adjust the value to get 5% overshoot. Change RL to 125Ω. Does the ringing get better or worse? Why?
An alternative method for frequency compensation of the regulator is shown above. What is the required value of the compensation capacitor C
c? Determine the value as before. At which load condition will you do this(R
L=∞ or R
L=125Ω) to cover all load conditions?
LM2940 is compensated in this manner.
Applications: Linear voltage regulators, such as the ones on your bench are made of circuits like this one. They also include the voltage reference generator. The “pass transistor” Q
4 is of sufficient rating for the maximum output current(You may be able to see large pass transistors mounted on heatsinks on the backside of some of the power supplies in the lab). Multiple buffer stages may be required(such as Q
3) to drive the base current of Q
4. Usually the feedback loops have more stages for more gain. For tracking dual power supplies, there is effectively another feedback circuit that looks like an inverting amplifier with the positive V
out as the input. On modern integrated circuits housing entire systems, like large portions of a radio, it is common to find even upto a dozen LDOs powering various blocks.
LM2940 is an example of a commercially available LDO. Page 12 of the datasheet has the schematic diagram.
The choice of a 1.2V reference is not arbitrary. A temperature stable voltage reference can be realized using bipolar transistors. Its output equals 1.2V(= bandgap of silicon extrapolated to absolute zero). For more information about this, see