Emitter Resistance In Widlar Current Source Calculator

Emitter Resistance Formula:

\[ Re = \frac{Vth}{Io} \times \log_{10}\left(\frac{Iref}{Io}\right) \]

Emitter Resistance (Re):

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1. What is Emitter Resistance in Widlar Current Source?

The Widlar current source is a modification of the basic two-transistor current source that incorporates an emitter resistor to achieve very low output currents. The emitter resistance (Re) helps in setting the precise current level by creating a voltage drop that controls the current flow through the output transistor.

2. How Does the Calculator Work?

The calculator uses the Widlar current source formula:

\[ Re = \frac{Vth}{Io} \times \log_{10}\left(\frac{Iref}{Io}\right) \]

Where:

\( Re \) — Emitter resistance (Ω)
\( Vth \) — Threshold voltage (V)
\( Io \) — Output current (A)
\( Iref \) — Reference current (A)
\( \log_{10} \) — Base-10 logarithm function

Explanation: The formula calculates the emitter resistance needed to achieve a specific output current in a Widlar current source configuration, using the logarithmic relationship between reference and output currents.

3. Importance of Emitter Resistance Calculation

Details: Accurate calculation of emitter resistance is crucial for designing precise current sources in analog integrated circuits. The Widlar configuration is particularly useful for generating very low currents that would be difficult to achieve with basic current mirror configurations.

4. Using the Calculator

Tips: Enter threshold voltage in volts, output current in amperes, and reference current in amperes. All values must be positive and non-zero. The output current should be less than the reference current for meaningful results.

5. Frequently Asked Questions (FAQ)

Q1: What is a Widlar current source used for?
A: Widlar current sources are used in integrated circuits to generate precise, low-value constant currents for biasing and other analog circuit applications.

Q2: Why is there a logarithmic function in the formula?
A: The logarithmic function accounts for the exponential relationship between base-emitter voltage and collector current in bipolar junction transistors.

Q3: What are typical values for emitter resistance in Widlar sources?
A: Emitter resistance values can range from a few ohms to several kiloohms, depending on the desired current levels and transistor characteristics.

Q4: How does temperature affect the calculation?
A: Temperature affects the threshold voltage and transistor characteristics. For precise applications, temperature compensation may be necessary.

Q5: Can this calculator be used for MOSFET Widlar sources?
A: While the principle is similar, MOSFET Widlar sources use different equations that account for MOSFET-specific parameters like transconductance.