In the realm of electronics, maintaining a consistent current is crucial for the stability and performance of many circuits. This is where the Widlar current source comes into play. Invented by the late Robert Widlar while working at National Semiconductor, this simple yet powerful circuit has been a staple in electronics design for decades. But what exactly is a Widlar current source, and how can you calculate its values? Let's dive in and explore this essential topic.
Imagine you're working on a project that demands a stable current source. You could use a fixed current sink, but that might not offer the precision or flexibility you need. Enter the Widlar current source, a variable current source that can provide a constant current to a load despite changes in voltage, making it ideal for driving LEDs, lasers, and other voltage-sensitive devices.
Understanding the Widlar Current Source
The Widlar current source is based on a simple principle: maintaining a constant voltage drop across a resistor to produce a stable current. At its heart lies an operational amplifier (op-amp) configured as a voltage follower, with its output connected to the base of an NPN transistor. The resistor in the emitter loop ensures a constant voltage drop, and thus, a stable current flow.
To create a Widlar current source, you'll need just a few components: an op-amp, an NPN transistor, a resistor for the emitter, and another resistor to set the current level. The simplicity and versatility of this circuit make it a fantastic addition to any electronics toolbox.
The Role of the Op-Amp
The op-amp in the Widlar current source acts as a buffer and an error amplifier. Its non-inverting input is connected to a reference voltage (Vref), while its inverting input is connected to the emitter of the NPN transistor via a voltage divider. The op-amp continually adjusts the output voltage to ensure that the voltage at the emitter (Vemitter) equals Vref.
By manipulating Vref, you can alter the current flowing through the circuit. For instance, if you want to double the current, you'd need to double Vref. This is because the current (I) is directly proportional to Vref (I = Vref / Re, where Re is the emitter resistor's resistance).
Setting the Current Level
The current level in a Widlar current source is primarily determined by the reference voltage (Vref) and the resistance in the emitter loop (Re). I = Vref / Re. To calculate the current, you'll first need to ensure that your op-amp's output can drive the transistor and that the voltage drop across Re is sufficient to maintain the desired current.
For example, let's say you want a current of 20mA and you've chosen a 10kΩ resistor for Re. To find Vref, rearrange the formula to Vref = I * Re, which gives you Vref = 20mA * 10kΩ = 200mV. So, you'd need a 200mV reference voltage to produce a 20mA current.
Enhancing the Basic Widlar Current Source
While the basic Widlar current source is effective, it has some limitations. For instance, the output current might not be perfectly proportional to the reference voltage, especially at low currents. There are, however, several ways to enhance the basic design and overcome these challenges.
Adding a BJT Current Mirror
One way to improve the Widlar current source is to add a BJT current mirror. This circuit consists of two NPN transistors with their bases connected together. The first transistor (Q1) acts as the current source, while the second (Q2) mimics the current flowing through Q1 and provides the output current.
The addition of a current mirror ensures that the output current is precisely proportional to the reference current, even at very low current levels. This makes the circuit more robust and reliable, making it ideal for applications that demand precise current control.
Adjustable Current Source
Another enhancement is to make the current source adjustable. By using a digital-to-analog converter (DAC) as the reference voltage source, you can programmatically control the output current. This is particularly useful in software-defined radio applications, where the transmit power needs to be adjustable.
To create an adjustable current source, simply replace the fixed voltage reference with a DAC. Ensure that the DAC's output range matches the required reference voltage (Vref) range. You can then control the current source's output by adjusting the DAC's input value.
In the world of electronics, the Widlar current source is truly a timeless innovation. Its simplicity, versatility, and effectiveness make it an essential tool for designers and hobbyists alike. Whether you're driving an LED, biasing an amplifier, or creating a stable current reference, the Widlar current source has you covered. So the next time you need a consistent current, don't uuidN over to the obvious solutions; look for the Widlar current source, and you'll be on the right track. Happy designing!