ADAPTIVE MOSFET RESISTOR
A MOSFET can operate as a resistor by operating in the linear or ohmic region of the drain V-I characteristics. This region can be obtained by floating the gate of the MOSFET, when the dc current and the voltage drop are given. Multiple resistors can be duplicated (or mirrored) by sharing the same source and floating gate. The floating gate voltage can be simulated using a closed loop equivalent circuit. Alternatively, the gate voltage can also be derived from the given drain-to-source voltage and the given current in a feedback loop. With this adaptive MOSFET resistor, the minimum supply voltage can be as low as the sum of the BJT threshold and the complementary BJT saturation voltage, e.g. VCC≧VBE+Vsat (e.g. 0.8+0.15<1.0V). The threshold voltage Vt should be less than VBE.
1. Field of the Invention
This invention relates bipolar-junction transistor (BJT)/MOSFET integrated circuit, particularly to resistive means to bias a BJT with a MOSFET.
2. Brief Description of Related Art
In integrated circuits, a polycrystalline film or diffusion is often used to fabricate resistors. Such resistors are limited in range of resistance values and may occupy substantial areas. Although it is well-known that a MOSFET can operate as a resistor in its ohmic or “triode” region of its drain V-I characteristics, it is seldom used in practice, because the resistance value varies with the gate voltage.
In circuit applications, a resistor is used to determine the current flow across a voltage differential, for instance between the supply voltage and an electrode of a transistor. For a BJT common emitter amplifier as shown in
If the dc base current is supply from a current mirror as shown in
In another instance as shown in
It is proposed here to use an adaptive gate MOSFET to serve as an ohmic resistor by operating the MOSFET is the linear region of the drain characteristics. The proposed adaptive gate MOSFET is to achieve the following objects:
-
- to adapt to a wide range of resistance value for any specified current and voltage drop;
- to require lower voltage drop than a current mirror, hence low supply voltage;
- to mirror the resistance to more than one resistance values;
- to float the gate of the MOSFET to adapt to the voltage across the resistor and the current through the resistor;
- to reduce the area occupied by the resistors in an IC.
These objects are achieved by using a MOSFET operating in the ohmic region of the drain V-I characteristic to bias the base of a BJT. The gate voltage of the MOSFET is adaptable, so the MOSFET resistor feeds the correct amount of dc base current to yield a desired dc collector current over a wide range of current amplification factor β. The adaptive gate voltage can be obtained by deriving the gate voltage from a current regulator or simply floating the gate.
BRIEF DESCRIPTION OF THE DRAWINGS
Operation of the MOSFET Resistor:
ID=kp(W/L)[(VGS−Vt)VDS−VDS2/2] (1)
where kp is a transconductance parameter, W/L is the width-to-length ratio of the gate, VGS is the gate to source voltage, Vt is the threshold voltage and VDS is drain-to-source voltage. When VDS is less than (VGS−Vt), the MOSFET is operating in the ohmic region, and
IC≈kp(W/L)(VGS−Vt)VDS (2)
The ohmic resistance is:
RDS=VDS/IC=1/[kP(VGS−Vt)] (3)
By adjusting VGS, a wide range of resistance values can be obtained, so long as VDS is less than (VGS−Vt).
Design of the Adaptive MOSFET Resistor:
Consider the pnp common emitter circuit
VB=(VCC−VBE.)<(VG−Vt) (4)
or (VCC−VBE+Vt)<VG (5)
Since VCC>VG (6)
Combining (5) and (6) yields:
VBE>Vt (7)
For IB>0, VB>0, as shown in
VCC>(VBE+Vsat)=0.8+0.15=0.95V
The current source Qp is mirrored from the master BJT Qp′, which is fed from an adaptable MOSFET resistor Mn described above.
The size of the MOSFET resistance can be calculated as follows:
IB=IC/β=ID=kp(W/L)(VCC−VG−Vt)(VCC−VB) (8)
W/L=(IC/β)/[kP(VCC−VG−Vt)(VCC−VB)] (9)
A medium value of gate voltage can be derived from Eq. (5),
VG=(VCC−VB)/2 (10)
This medium VG can allow a IC/β variation twice that of the medium value.
Derivation of the Gate Voltage:
The gate voltage for the MOSFET resistor can be obtained as follows: The gate voltage can be derived with a regulator circuit as shown in
Floating Gate Operation:
When the voltage across the MOSFET resistor and the current are known, there is only one gate voltage for the MOSFET resistor to satisfy this condition. Thus, if the voltage across the MOSFET is given and the current is given, the gate automatically adjusts itself, even if it is floating. This feature can sometimes be utilized as illustrated in
A circuit to demonstrate this floating gate resistor is shown in
This resistance can be duplicated (mirrored) in other resistors on the same chip. For instance in
If the values of the MOSFET resistors shown in
To simulate the floating MOSFET, a dc gate voltage must be assigned to the floating gate. In a typical simulation program such as SPICE, the program does not run with the gate floating. The floating gate voltage can be derived from the regulated gate voltage derived from
Layout of the Floating Gate Resistor:
The floating gate MOSFETs such as M7, M8′ can share a common source and a common floating gate, but different drains D7, D7′, D8′. A layout is shown in
In the foregoing descriptions of the current mirrors in
While the preferred embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications can be made without departing the spirit of the present invention. Such modifications are all within the scope of the present invention.
Claims
1. An ohmic resistor comprising:
- MOSFET having a drain, a source and a gate;
- a dc voltage applied across said MODFET between said rain and said source;
- a dc drain current flowing from the drain to the source; and
- an adaptive gate voltage appearing automatically at said gate such that the MOSFET operates in the ohmic region of the drain V-I characteristic.
2. The ohmic resistor as described in claim 1, wherein nothing is connected to said gate.
3. The ohmic resistor as described in claim 2, further comprising resistance mirror to mirror the resistance of said ohmic resistance, comprising:
- more than one MOSFETs having a common source, a common floating gate and separate drains.
4. The ohmic resistor as described in claim 3, wherein said drains are connected to separate load devices.
5. The ohmic resistor as described in claim 3, wherein the channels of said MOSFETs are isolated from each other with channel stoppers.
6. The ohmic resistor as described in claim 1, wherein the drain of said MOSFET (e.g. pMOS) is connected to the base of a complementary type bipolar junction transistor (e.g. npn BJT), operating as a common emitter amplifier.
7. The ohmic resistor as described in claim 1, wherein said MOSFET is an NMOS having a drain connected to a pnp BJT current mirror and the output of the current mirror driving the base of an npn common emitter amplifier.
8. The ohmic resistor as described in claim as described in claim 1, wherein said ohmic resistor is connected to the first base of a BJT differential pair fed from a current source, and a reference voltage connected to the second base of said differential pair.
9. The ohmic resistor as described in claim 6 further comprising additional BJT differential pairs having said ohmic resistor connected to each base of said additional differential pairs.
10. The ohmic resistor as described in claim 5, wherein the width ratio of the separate drains is varied to vary inversely the resistance ratio of the resistance mirror.
11. The ohmic resistor as described in claim 1, wherein said gate is applied with a regulating gate voltage derived from said drain-to-source voltage and said drain current so as to satisfy the drain voltage vs current characteristic of a MOSFET.
12. The ohmic resistor as described in claim 11, wherein said regulating gate voltage is derived from a feedback loop.
13. The ohmic resistor as described in claim 12, wherein the source current from a MOSFET applied with a specified dc drain-to source voltage is compared with a reference source current and a compared error output voltage is fed back to the gate of the MOSFET to constitute said adaptive gate voltage for regulating the source current from said MOSFET and the reference source current to be equal.
Type: Application
Filed: Oct 20, 2003
Publication Date: Apr 21, 2005
Inventor: Hung Lin (Silver Spring, MD)
Application Number: 10/688,437