Abstract
In this article, a new bias current regulation circuit of a one-stage bipolar and Filed Effect Transistor amp is described. How the FET achieves high precision and simplicity by means of a combination of feedback error amplifier with a bandgap reference is explained.
Introduction
Regulating current through active devices sets the output level in a drive circuit. Figure 1. Below shows typical application of these active devices. Qo I used as a FET rather than a BJT. In a limiting amp, the Qo acts as a switch and the output signal level is proportional to the bias current. The gain of linear amp depends on current Io through Qo. Precise regulation improves performance and maintenance. Power consumption is a key concern in low voltage circuits. The power efficiency of a circuit is reduced due to voltage drop across resistor Rs (Banwell 124).
Figure 1. common circuits where the point of operation is set by (a) linear or limiting amp, (b) laser or line driver, (c) high power laser bias circuit
Two conventional current sensing feedback topologies are used in the circuit to implement circuits in figure 1. Q1 and Q2 compare voltage drop IoRs with a reference source Vref.
New approach
In figure 2 above, a departure from conventional practice in circuit 2(a) is made when Vref is much less than 1 volt where variation in bias voltage is controlled by regulating the ratio of collector currents I1 and I2. Also, R1 and R3 are added to convert amps Q1 and Q2 into bandgap derived reference shown in 2 (b). The divider R1 and R2 scales the bias voltage with Q3 feedback where R3 provides the base current compensation (Leighton 217). Assuming that a closed gain is large, the nodal analysis of this circuit becomes;
1
Where is the ratio of transistors Q1 and Q2 emitter current densities, m is the actual ratio of transistor emitter areas. is thermal voltage, is the forward ideality factor. Overall complexity of and performance of the circuit in figure 1 achieved in the circuit in figure 2 (b) depends on the intermediate amp. Q2 in figure 2 (b) provides appreciable gain. The regulation of I1 and I2 which is the stability of Io, is determined by the voltage gain and intrinsic supply rejection of the intermediate amplifier and the common base voltage gain provided by Q2. The overall current regulation of circuits 1 (a) and 1 (b) is described as:
..2
Where and are the overall amp gain with and without loading of the Qo respectively. In equation 2, go0 is output conductance of Qo. The current gain depends on the number (n) of transistors in the path excluding common base stages. denotes the current gain of Qo.
Collector voltage Vc2 varies as the feedback loop compensates for both temperature and supply voltages. The resultant deviation in Q2’s emitter voltage from reference voltage produces apparent variation in the reference voltage. With ideal current sources providing I1 and I2, the following illustration can be written,
..3
Where is the early voltage, is the trans-conductance of transistor Q2, and is the dynamic input resistance in at intermediate stage.
Finally, the work done by reviewers was greatly appreciated for their critical comments that led to substantial improvement of the manuscript.
Figures
Figure 1: Common circuits where the point of operation is set by (a) linear or limiting amp, (b) laser or line driver, (c) high power laser bias circuit
Figure 2: Conventional current sensing feedback topologies (a), (c). Proposed improved feedback amp topologies with internal low voltage band gap reference (b), (d).
Work cited
L. Leighton, “AGC for the VMOS RF power amplifier: Application Note ANSO-6,” in MOSPOWER Design Catalog, Siliconix Inc., Santa Clara, CA, 1983, pp. 5.77-5.79.
T. C. Banwell, A. C. Von Lehmen, and R. R. Cordell, “VCSE laser transmitters for parallel data links,” IEEE J. Quantum Electron., to be published.
