A balanced cascode current mirror

Abstract: A balanced cascode current mirror includes a first and second current paths respectively defined by first and second transistors and by third and fourth transistors. Each current path may include the sources and drains of the transistors in such path. Connections may respectively extend between the gates of the first and third transistors and between the gates of the second and fourth transistors to provide the first and third transistors with substantially identical source, gate, and drain impedances. An input current is introduced to the drain of the second transistor and an output current with substantially identical characteristics is obtained from the drain of the fourth transistor. A capacitance may be connected between the drain of the second transistor and the gate of the first transistor to produce a flow of a current at high frequencies through the first current path corresponding to the input current at the drain of the second transistor. A fifth transistor may be connected in a circuit with a constant current source to regulate the current through the first and second transistors to be substantially equal to the input current at low frequencies. The gate of the fifth transistor may be connected to the drain of the second transistor, and the source of the fifth transistor may be connected to the gate of the first transistor, to provide this current regulation. The gates of the second and fourth transistors and the drain of the fifth transistor may have a common reference potential such as ground.

Description: This invention relates to current mirrors. More particularly, the invention relates to a balanced cascode current mirror according to the preamble part of claim 1. Current mirrors are used often in electronic systems to produce an output current corresponding to an input current. The current mirrors are provided with two (2) branches or current paths. The input current is introduced to one branch, or current path, the branches or current paths being interconnected to produce the output current at the second branch or current path. The output from the second branch or current path is often introduced to an amplifier to provide amplification. The current mirrors now in use have been found to have limitations which become aggravated with increases in frequency. This results from the fact that the impedances of the individual transistors in the first path do not match the impedances of the corresponding transistors in the second path. The differences in the impedances between the transistors in the first and second branches result from the fact that the transistors in the first path are connected to operate as diodes whereas the transistors in the second path operate as common-source amplifiers. Since the frequencies of electrical circuits on integrated circuit chips have been progressively increasing with time, the differences between the characteristics of the output current relative to the input current in current mirrors have become progressively serious. Attempts have been made to resolve such problems but with limited success. The object of the present invention is to provide a current mirror circuit having a balanced relationship between the impedances in the two current paths. This invention provides a current mirror having the features of patent claim 1. It provides two branches or current paths, one for the input current and the other for the output current, and provides a balanced relationship between the impedances in the two current paths. Furthermore, it provides for a balanced response of the two current paths to changes in the input current at high frequencies. It also provides for a good balance of the current between the first current paths at low frequencies. In this way, the output current in the second current path corresponds to the input current in the first current path through a range of frequencies considerably greater than in the prior art. Preferred embodiments are subject to various dependent claims. In the drawings: Figure 1 is a circuit diagram of a current mirror of the prior art; Figure 2 is a simplified circuit diagram of a balanced cascode current mirror constituting one embodiment of the invention; and Figure 3 is a circuit diagram of the balanced cascode current mirror of Figure 2 with a transistor in Figure 3 functioning as a capacitor in Figure 2. Figure 1 shows a current mirror, generally indicated at 100, of the prior art. The current mirror is generally indicated at 100 and is shown as comprising four (4) transistors 101, 102, 103 and 104. These transistors are shown in Figure 1 as being formed by a CMOS process and as constituting p types of transistors. However, it will be appreciated that the transistors may be n- type transistors fabricated by the CMOS process or that they may be fabricated by other processes such as the bipolar process. In the current mirror 100 shown in Figure 1, the sources of the transistors 101 and 102 may be connected to a source 106 of positive voltage. The gate and the drain of the transistor 101 are common with the source of the transistor 103 and the gate of the transistor 102. The gate and the drain of the transistor 103 have a common connection with a line 107 and with the gate of the transistor 104. The drain of the transistor 102 is connected to the source of the transistor 104 and the drain of the transistor 104 is connected to a line 110 for providing a current output. In the circuit shown in Figure 1, the input current introduced to the line 107 flows through a current path defined by the transistors 101 and 103. The resultant voltages produced on the drain and gate of the transistor 101 and on the drain and gate of the transistor 103 are respectively introduced to the gates of the transistors 102 and 104. This theoretically causes the current output on the line 110 to match the current input on the line 107. The current output on the line 110 may be introduced to an amplifier stage (not shown). There are several inherent disadvantages with the current mirror shown in Figure 1, particularly at high frequencies. This results from the fact that the current mirror is not particularly well balanced at high frequencies. For example, the drain and gate of the transistor 101 are common but the drain and gate of the transistor 102 are not common. Similarly, the drain and gate of the transistor 103 are common but the drain and gate of the transistor 104 are not common. This causes the impedance seen by the drain of the transistor 101 at high frequencies to be considerably less than the impedance seen by the drain of the transistor 102 at such high frequencies. This impedance difference will cause the current balance of the current mirror 100 to be upset at high frequencies. Another problem with the current mirror 100 at high frequencies is that the input current on the line 107 passes initially through the impedance of the transistor 103 before charging the gate capacitances of the transistors 101 and 102. For a particular phase lag in the current mirror 100, the presence of the transistor 103 causes the frequency to be at least a factor of two (2) lower than if the transistor 103 were not in the current path between the current input on the line 107 and the gate of the transistor 101. Figure 2 provides a balanced cascode current mirror, generally indicated at 200, which constitutes one embodiment of the invention and which overcomes the disadvantages discussed above with respect to the prior at embodiment shown in Figure 1. In the embodiment shown in Figure 2, the current input is provided on the line 107 and the current output is obtained from the line 110 as in the circuit shown in Figure 1. The embodiment shown in Figure 2 further includes transistors 201, 202, 203 and 204 which may be fabricated by the CMOS process and may comprise p-type transistors. However, as in the circuit shown in Figure 1, the transistors may be fabricated by other processes and may be other than p-type transistors. The embodiment shown in Figure 2 includes a supply 106 of positive voltage. This supply is connected to the sources of the transistors 201 and 202. The drain of the transistor 201 has a common connection with the source of the transistor 203. The drain of the transistor 205 is connected to a source of reference potential such as ground. In like manner, the drain of the transistor 202 has a common connection with the source of the transistor 204. The drain of the transistor 204 is common with the line 110 providing the current output. The gates of the transistors 203 and 204 have a common connection with a reference potential such as ground. A connection is made from the drain of the transistor 203 to the gate of transistor 205 and one terminal of a capacitor 206, the other terminal of which is common with the gates of the transistors 201 and 202. The drain of the transistor 203 is also connected to the gate of a transistor 205, the drain of which is at the reference potential such as ground. The source of the transistor 205 is connected to the gates of the transistors 201 and 202. A source 207 of constant current is connected between the source of the transistor 205 and the voltage source 106. Current flows through the current path defined by the transistors 201 and 203 in accordance with the current input on the line 107. The current flow through the transistors 201 and 203 is facilitated at high frequencies by the capacitance 206 which couples fast changes in the current input on the line 107 directly to the gates of the transistors 201 and 202 without requiring that these fast changes pass through other components. For example, when the current input on the line 107 increases rapidly, the voltage introduced through the capacitance 206 to the gates of the transistor 201 and 202 decreases rapidly and causes the currents through the transistors 201 and 202 to increase. In this way, the current changes in the path defined by the transistors 201 and 203 and in the path defined by the transistors 202 and 204 match the changes in the current input on the line 107 at high frequencies. The change in the current paths is facilitated by the fact that the transistors 201 and 202 respectively see low load impedances. The load impedances are low because they are provided by the transistors 203 and 204 with the grounded gates. As will be appreciated, the current flowing through the path defined by the transistors 202 and 204 is equal to the current flowing through the transistors 201 and 203 because the impedances of the transistors 202 and 204 respectively match the impedances of the transistors 201 and 203. The current mirror shown in Figure 2 also operates advantageously at low frequencies and at DC. This results from the operation of the transistor 205 in providing a low frequency coupling of the voltage on the current input line 107 from the gate of the transistor 205 to the source of the transistor 205. The voltage on the source of the transistor 205 is in turn introduced to the gates of the transistors 201 and 202 to produce a flow of current through the paths defined by the transistors 201 and 203 and by the transistors 202 and 204 in accordance with the current input on the line 107. The operation of the transistor 205 may be further seen from a specific example. Assume that, at low frequencies, the voltage on the gate of the transistor 201 is too positive to produce a sufficient flow of current through the transistors 201 and 203 to match the current input on the line 107. Since the current flow through the transistor 201 is low, the current through the transistor 203 is also low. This causes the voltage on the drain of the transistor 203 and the gate of the transistor 205 to be low. Because of the constant current through the transistor 205 as a result of the operation of the current source 207, the voltage on the source of the transistor 205 decreases in accordance with the decrease in the voltage on the gate of the transistor 205. The decreased voltage on the source of the transistor 205 is introduced to the gates of the transistors 201 and 202 to produce an increase in the currents through these transistors. In this way, the transistor 205 provides a servo path at low frequencies to regulate the current through the path defined by the transistors 201 and 203, and the path defined by the transistors 202 and 204, in accordance with the current input on the line 107. The circuit shown in Figure 3 is identical to the circuit shown in Figure 2 except that it includes a transistor 306 instead of the capacitance 206. The gate of the transistor 306 is connected to the drain of the transistor 203 and the source and the drain of the transistor 306 are connected to the gates of the transistors 201 and 202. The transistor 306 operates in a manner similar to that described above for the capacitor 206. The current mirror constituting this invention has certain important advantages over the prior art. One advantage is that it is well balanced. Another advantage is that it includes cascode transistors for providing matched low impedances in each of the input and output current paths. A further advantage is that the current mirror responds rapidly to changes at high frequencies in the current input on the line 107. In addition, the current through the path defined by the transistors 201 and 203 and the path defined by the transistors 202 and 204 is regulated at low frequencies to assure that the currents through these paths match the current input on the line 107.

Claims: Current mirror circuit comprising: a first current path including first (201) and second (203) transistors, first means (107) for introducing an input current to the second transistor (203) to obtain a flow of this current through the first current path, a second current path including third (202) and fourth (204) transistors, second means for connecting the first (201) and third (202) transistors and the second (203) and fourth (204) transistors to provide an impedance in the third transistor (202) matching the impedance in the first (201) transistor and to provide an impedance in the fourth transistor (204) matching the impedance in the second transistor (203) and to provide a flow of current through the second current path corresponding to the flow of current through the first current path, characterized by third means (206,306) for coupling the first (201) and second (203) transistors to provide for a high frequency biasing of the first transistor (201) in accordance with the input current introduced to the second transistor (203), to regulate the flow of input current at high frequencies through the first (201) and second (203) transistors in accordance with the input current introduced to the second transistor (203), and fourth means (205,207) for coupling the first (201) and second (203) transistors to provide for a low frequency biasing of the first transistor (201) to regulate the flow of the input current at low frequencies through the first (201) and second (203) transistors in accordance with the input current introduced to the second transistor (203). Circuit as set forth in claim 1, wherein the third means include a capacitor (206) coupling the first (201) and second (203) transistors at high frequencies to provide for the flow of the input current through the first current path. Circuit as set forth in claims 1 or 2, wherein the fourth means include a fifth transistor (205) biased in accordance with differences between the input current and the current through the first (201) and second (203) transistors to regulate the current through the first and second transistors to conform to the input current. Circuit as set forth in claim 3, wherein the fourth means further includes a constant current source (207) providing a substantially constant current through the fifth transistor (205). Circuit as set forth in at least one of claims 1 to 4 wherein the third means (206) are operative to introduce changes at high frequencies in the voltage from the second transistor (203) to the first transistor (201) to regulate the current through the first (201) and second (203) transistors in accordance with the current introduced to the first current path. Circuit as set forth in at least one of claims 1 to 5, wherein the fourth means (205,207) are operative to introduce changes at low frequencies in the voltage from the second transistor (203) to the first transistor (201) to regulate the current through the first (201) and second (203) transistors in accordance with the current introduced to the first current path. Circuit as set forth in at least one of claims 1 to 6, wherein the gate electrodes of the first (201) and third (202) transistors are connected with each other and coupled to the third and fourth means (205,206,207) to provide the bias for the first and third transistors (201, 202). Circuit as set forth in at least one of claims 1 to 7, wherein the gate electrodes of the second (203) and fourth (204) transistors are commonly connected to a reference bias potential. Circuit as set forth in at least claim 1, wherein said third means include a transistor (306), coupled to the first (201) and second (203) transistor to provide for the flow of input current through the first current path. Circuit as set forth in at least one of claims 1 to 9, wherein the first to fourth transistors are p-type CMOS transistors, wherein the drain electrodes of the first (201) and third (202) transistors are coupled to the source electrodes of the second (203) and fourth (204) transistors, respectively. Circuit as set forth in claim 10, wherein the fourth means is a p-type CMOS transistor (205) having its source electrode coupled to the gate electrodes of the first (201) and third (202) transistors and its gate electrode coupled to the drain electrode of the second transistor (203).

Inventors: LEWYN LENNY L, LEWYN, LENNY L.
Applicants: BROOKTREE CORP, BROOKTREE CORPORATION
Application year: 1991
Main classification: G05F3
IPC codes: G05F3, H03F3, H03F1