9.1. Basic Current Source
An example of a current source based on PMOS transistors is shown here in Fig. 9.1. It consists of the reference circuit, consisting of transistor M3 and bias resistor Rbias. The reference-circuit transistor is diode connected. The induced VSG3 = VSD3 is applied to the current-source transistor M2. Thus, the drain current, or current-source current of M2 is the mirror of the reference current. In an integrated circuit, any number of current sources can be referenced to the reference current or voltage. Current ratios are implemented with the selection of the relative gate widths of the transistors.
Figure 9.1. PMOS current source. The current set up by the reference circuit of M3 and Rbias is mirrored as the current of M2.
The design of the reference circuit is based on a dc solution to the reference current, ID3. The solution is obtained from the loop equation
Equation 9.1
and transistor equation
Equation 9.2
The current in M2 is
Equation 9.3
where
and
W2 and W3 are the transistor gate widths of M2 and M3, respectively, and L is the channel length. In general, the ratio of currents is
Equation 9.4
where the approximation can often be used for simplicity with long-channel devices. This avoids the necessity of knowing the source – drain voltages. The signal output resistance at the drain of M2 is
Equation 9.5
In the project of the current source, we will scan VSD2 and compare the source current with the reference current.