High speed low offset CMOS amplifier with power supply noise isolation

Abstract: A single-ended amplifier circuit is provided combining high speed operation with low input offset characteristics while providing immunity from power supply noise for the input signal by isolating the power supply conductors (22, 26) from the input conductor. The first (16) and second (18) amplifying transistors are separated from the power supply conductors by the drain-source conduction paths of first (20) and second (24) isolation transistors operating in their linear region in response to a predetermined reference potential. Thus, any power supply noise is absorbed across the third and fourth isolation transistors thereby avoiding overwhelming a low level input signal with switching induced noise.

Description: Field of the Invention This invention relates in general to a high frequency low offset amplifier providing isolation for a low level input signal from power supply noise. Background of the Invention Amplifier circuits are commonplace in modern electronic design. Some desirable features for a single-ended amplifier circuit include high speed operation with a low input offset voltage. For example, the specifications of the amplifier may require operation in the Intermediate Frequency (IF) band with an input signal in the millivolt range. It is desirable to have a substantially zero input offset voltage for the amplifier with such low level input signals. Otherwise, the input signal may not reach a sufficient level to activate the output signal of the amplifier. Another consideration for an amplifier operating with small input signals is the noise level on the power supply conductors, wherein the power supply noise may feed through and overwhelm the low level input signal. Given such restrictive specifications, the prior art has typically employed linear bipolar technology to achieve the low offset voltage and power supply noise isolation since single-ended MOS amplifiers with low level input signals historically have been susceptible to noise. In many applications, it is necessary to amplify the low level analog input signal to digital levels for further processing through digital circuits. The digital circuits are typically implemented with MOS technology because of the low power consumption while high speed linear circuits are bipolar in design. Thus, conventional practice has taught either using separate bipolar and MOS integrated circuit (IC) chips for the amplifier and digital logic functions, respectively, thereby maintaining a single technology per IC, or combining bipolar and MOS technologies onto a single application specific integrated circuit (ASIC). However, both of these options increase the overall cost as compared to a single technology design for both the linear and digital circuits on one integrated circuit. Hence, what is needed is an improved single-ended amplifier circuit implemented in MOS technology combining high speed operation with a low offset voltage for low level input signals while providing noise isolation from the power supply conductors. Summary of the Invention Briefly stated, in accordance with the present invention there is provided a MOS amplifier circuit having an input coupled for receiving an input signal and an output for providing an output signal, the amplifier circuit being characterized by: first means having an input coupled to the input of the amplifier circuit for amplifying the input signal and having an output for providing the output signal, said first means including a first terminal coupled for receiving a first operating potential and a second terminal coupled for receiving a second operating potential, said first means comprising, a first transistor having a gate, a drain, and a source, said gate coupled to said input of said first means, said source being coupled to said first terminal of said first means, said drain being coupled to said output of said first means, a second transistor having a gate, a drain and a source, said gate being coupled to said input of said first means, said source being coupled to said second terminal of said first means, said drain being coupled to said output of said first means; second means coupled between a source of said first operating potential and said first terminal of said first means and operating in a linear region in response to a reference potential for reducing the influence of the voltage noise of said first operating potential, said second means including a third transistor having a gate, a drain and a source, said gate being coupled to said reference potential, said source being coupled to the first source of operating potential, and said drain being coupled to said first terminal of said first means; third means coupled between a source of said second operating potential and said second terminal of said first means and operating in a linear region in response to a reference potential for reducing the influence of the voltage noise of said second operating potential, said third means including a fourth transistor having a gate, a drain and a source, said gate being coupled to said reference potential, said source being coupled to the second source of operating potential, and said drain being coupled to said second terminal of said first means; and feedback means coupled between said output and said input of said first means for controlling the amplification of said first means. The present invention includes a single-ended amplifier circuit implemented in MOS technology combining high speed operation and low input offset characteristics while providing immunity from power supply noise for the input signal by isolating the input signal from the power supply conductors. Preferably, the amplifier circuit further comprises circuit means for providing a reference potential to the gates of the first and second isolation transistors of the second and third means respectively whereby the first and second isolation transistors operate in their linear region. Thus, the first and second amplifying transistors are separated from the power supply conductors by the drain-source conduction paths of first and second isolation transistors operating in their linear regions in response to a predetermined reference potential. Thus, any power supply noise is absorbed across the first and second isolation transistors thereby avoiding overwhelming a low level input signal with switching induced noise. Brief Description of the Drawing The sole figure is a schematic diagram illustrating the preferred embodiment of the present invention. Detailed Description of the Preferred Embodiment Referring to the sole figure, there is shown single-ended amplifier circuit 10 suitable for manufacturing in monolithic integrated circuit form using conventional MOS integrated processes. A low level analog input signal, say 20 millivolts, is applied at input 12 of amplifier section 14 at the gates of transistors 16 and 18. The source of transistor 16 is coupled through the drain-source conduction path of transistor 20 to power supply conductor 22 typically operating at a positive potential such a VDD. Likewise, the source of transistor 18 is coupled through the drain-source conduction path of transistor 24 to power supply conductor 26 typically operating at ground potential. The drains of transistors 16 and 18 are coupled together at node 30 that is the output of amplifier section 14. Node 30 is also coupled through feedback resistor 32 to the gates of transistors 16 and 18 for controlling the gain of amplifier section 14. The gates of transistors 20 and 24 are responsive to a reference potential developed at node 34 by way of the resistor divider network formed of resistors 38 and 40 coupled between power supply conductors 22 and 26. Capacitor 42 is coupled between node 34 and power supply conductor 26 for filtering the high frequency noise feeding through transistors 20 and 24 to node 34. The output of amplifier section 14 may also be coupled to the input of amplifier section 46 at the gates of transistors 48 and 50. The source of transistor 48 is coupled to power supply conductor 22, while the source of transistor 50 is coupled to power supply conductor 26. The drains of transistors 48 and 50 are coupled together through feedback resistor 52 to the gates thereof for limiting the gain of amplifier section 46. The output of amplifier circuit 10 is provided at node 54 at the drains of transistors 48 and 50. The substrate of the integrated circuit comprising amplifier circuit 10, that is power supply conductor 26, often includes hundreds of millivolts of power supply noise principally due to high speed switching currents from surrounding digital logic circuits disposed on the same substrate (not shown). Similar noise may appear on power supply conductor 22. Most if not all conventional single-ended CMOS amplifiers have been unable to isolate the power supply noise from the input conductor, thus limiting the low operative level input signal. As part of the present invention MOS transistors 20 and 24 are coupled between power supply conductor 22 and 26 and transistors 16 and 18, respectively, for providing isolation therebetween. A reference potential is developed at node 34 of say 2.5 volts from the resistor divider combination of resistors 38 and 40. Capacitor 42 filters the high frequency components at node 34 for providing a stable reference signal at the gates of transistors 20 and 24. Hence, transistor 20 is continuously operating in its linear region for providing isolation between power supply conductor 22 and the source of transistor 16. Likewise, transistor 24 also operates in its linear region for providing isolation between power supply conductor 26 and the source of transistor 18. A typical drain-source resistance for transistors 20 and 24 is 200 ohms with 200 millivolts of potential thereacross. Any switching induced noise appearing on power supply conductors 22 and 26 is substantially absorbed across the drain-source conduction path of transistors 20 and 24 thereby removing such interference from the gates of transistors 16 and 18. Transistors 16 and 18 are operated in saturation such that the analog input signal applied at input 12 is amplified through amplifier section 14 to node 30. Feedback resistor 32 is provided for limiting the open loop gain and improving the stability of amplifier section 14. A typical value for resistor 32 is 15K ohms. Thus, with an input impedance seen at the gates of transistors 16 and 18 of approximately 300 ohms, amplifier section 14 provides a close loop gain of approximately 50 (15K/300). The high speed, low power operation is inherit in the CMOS design and the low input offset voltage is provided by biasing transistors 16 and 18 in the mid-range between power supply conductors 22 and 26. The output signal of amplifier section 14 is applied at the input of amplifier section 46 for providing additional amplification to drive the analog input signal to the rails of power supply conductors 22 and 26 thereby converting it to a digital signal at node 54. Transistors 48 and 50 also operate in saturation while feedback resistor 52 limits the gain of amplifier section 46. The isolation transistors like 22 and 24 are omitted from amplifier section 46 since the magnitude of the signal appearing at node 30 is well above the noise floor of power supply conductors 22 and 26. It is understood that amplifier sections like 14 may be cascaded after node 30. The same is true of amplifier section 46. Furthermore, inverter circuits formed of transistors like 48 and 50 without resistor 52 may be cascaded after node 54 for providing buffering of the output signal of amplifier circuit 10. The additional amplifier stages are also biased in the mid-range between power supply conductors 22 and 26 for providing high speed operation and amplification. Hence, what has been provided is a novel amplifier circuit implemented in MOS technology having high speed operation and low input offset characteristics while providing immunity from power supply noise for the input signal by isolating the power supply conductors from the input conductor.

Claims: A MOS amplifier circuit having an input coupled for receiving an input signal and an output for providing an output signal, the amplifier circuit being characterized by: first means (16,18) having an input coupled to the input of the amplifier circuit for amplifying the input signal and having an output for providing the output signal, said first means including a first terminal coupled for receiving a first operating potential and a second terminal coupled for receiving a second operating potential, said first means comprising, a first transistor (16) having a gate, a drain, and a source, said gate coupled to said input of said first means, said source being coupled to said first terminal of said first means, said drain being coupled to said output of said first means, a second transistor (18) having a gate, a drain and a source, said gate being coupled to said input of said first means, said source being coupled to said second terminal of said first means, said drain being coupled to said output of said first means; second means (20) coupled between a source of said first operating potential and said first terminal of said first means and operating in a linear region in response to a reference potential for reducing the influence of the voltage noise of said first operating potential, said second means including a third transistor (20) having a gate, a drain and a source, said gate being coupled to said reference potential, said source being coupled to the first source of operating potential, and said drain being coupled to said first terminal of said first means; third means (24) coupled between a source of said second operating potential and said second terminal of said first means and operating in a linear region in response to a reference potential for reducing the influence of the voltage noise of said second operating potential, said third means including a fourth transistor (24) having a gate, a drain and a source, said gate being coupled to said reference potential, said source being coupled to the second source of operating potential, and said drain being coupled to said second terminal of said first means; and feedback means (32) coupled between said output and said input of said first means for controlling the amplification of said first means. The amplifier circuit of claim 1 wherein the amplifier circuit further comprises: circuit means (38-42) for providing a reference potential to the gates of the third (20) and fourth (24) transistors of the second and third means respectively whereby the third and fourth transistors operate in their linear region, the circuit means comprising: first (38) and second (40) resistors coupled in series at a node (34) between the first source of operating potential and the second source of operating potential, the node (34) being coupled to the gates of the third (20) and fourth (24) transistors; and a capacitor (42) coupled between the node (34) and the second source of operating potential. A amplifier circuit according to any preceding claim wherein the amplifier circuit is integrated on an integrated circuit.

Inventors: MAHABADI JOHN K, MAHABADI, JOHN K.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1991
Main classification: H03F1
IPC codes: H03F1, H03F3