AMPLIFIER BIAS CIRCUIT

Abstract

Methods and apparatus for an amplifier including first and second transistors coupled in a stacked configuration with first and second current mirrors to provide respective bias signals to the amplifier transistors. A reference transistor is coupled to the first and second current mirrors for referencing the bias signals together.

Claims

1. A circuit, comprising: an amplifier including first and second transistors coupled in a stacked configuration; a first current mirror having a first control loop and a first mirror transistor coupled to a first terminal of the first transistor to provide a first bias current; a second current mirror having a second control loop and a second mirror transistor coupled to a first terminal of the second transistor to provide a second bias current; and a reference transistor coupled to the first and second current mirrors.

2. The circuit according to claim 1, wherein the first current mirror includes the first mirror transistor, a first fixed current source, and a first follower transistor coupled to the first mirror transistor in a follower configuration.

3. The circuit according to claim 2, wherein the second current mirror includes the second mirror transistor, a second fixed current source, and a second follower transistor coupled to the second mirror transistor in a follower configuration.

4. The circuit according to claim 1, wherein the reference transistor is coupled to the first mirror transistor and the second mirror transistor.

5. The circuit according to claim 1, wherein the first current control further includes at least one diode.

6. The circuit according to claim 1, wherein an output of the first current mirror is configured to establish a source potential for a common gate reference of the second current mirror.

7. The circuit according to claim 6, wherein the common gate reference of the second mirror has a drain current from the second fixed current source.

8. The circuit according to claim 1, wherein the amplifier comprises a depletion mode FET amplifier.

9. The circuit according to claim 1, wherein the amplifier comprises an RF amplifier.

10. A method, comprising: employing an amplifier including first and second transistors coupled in a stacked configuration; employing a first current mirror having a first control loop and a first mirror transistor coupled to a first terminal of the first transistor to provide a first bias current; employing a second current mirror having a second control loop and a second mirror transistor coupled to a first terminal of the second transistor to provide a second bias current; and employing a reference transistor coupled to the first and second current mirrors.

11. The method according to claim 10, wherein the first current mirror includes the first mirror transistor, a first fixed current source, and a first follower transistor coupled to the first mirror transistor in a follower configuration.

12. The method according to claim 11, wherein the second current mirror includes the second mirror transistor, a second fixed current source, and a second follower transistor coupled to the second mirror transistor in a follower configuration.

13. The method according to claim 10, wherein the reference transistor is coupled to the first mirror transistor and the second mirror transistor.

14. The method according to claim 10, wherein the first control loop further includes at least one diode.

15. The method according to claim 10, wherein an output of the first current mirror is configured to establish a source potential for a common gate reference of the second current mirror.

16. The method according to claim 15, wherein the common gate reference of the second mirror has a drain current from the second fixed current source.

17. The method according to claim 10, wherein the amplifier comprises a depletion mode FET amplifier.

18. The method according to claim 10, wherein the amplifier comprises an RF amplifier.

19. A circuit comprising: an input terminal to receive an RF input signal; an output terminal to output an RF output signal; and a means for amplifying the RF input signal and generating the RF output signal.

Description

DETAILED DESCRIPTION

[0023] FIG. 4 shows an example amplifier 400 having an input 402 and an output 404. Example embodiments of the disclosure may be well-suited for RF amplifier applications. The amplifier 400 includes a first transistor 406 and a second transistor 408 which may be configured in a cascode/stacked arrangement coupled in series from a voltage supply Vdd1 to ground.

[0024] In embodiments, the first transistor 406 is biased by a first bias signal Vg1 410 and the second transistor 408 is biased by a second bias signal Vg2 412. In the illustrated embodiment, first and second current mirrors are used to provide the bias signals to the first and second transistors 406, 408.

[0025] A first mirror transistor 420 is coupled to the first transistor 406 of the amplifier and a second mirror transistor 422 is coupled to the second transistor 408 of the amplifier. The first and second mirror transistors 420, 422 provide first and second control loops, as described more fully below, for a bias circuit to set amplifier quiescent bias. The first mirror transistor 420 and a first fixed current reference 423 in a first control loop 425 establish quiescent in the first (common source FET) transistor 406 of the amplifier. A voltage output of the first mirror transistor 420 goes to a reference transistor 424 for establishing the source potential for the common gate reference in the second mirror transistor 422. In embodiments, the second mirror transistor 422 (the common gate reference FET in the second mirror circuit) has a drain current from a second fixed current reference 426. The output of the second current mirror 422 may go to the second transistor 408 (top or common gate FET in the cascode/stacked FET network).

[0026] FIG. 5 shows a prior art circuit 500 for a depletion mode cascode amplifier. Typically, quiescent conditions are set by directly applying voltages Vg1 and Vg2 to the stacked FETs Q1, Q2 from DC supplies. FIG. 6 shows a prior art circuit having a separate voltage divider circuit to derive Vg2 and Vg1. In these conventional arrangements, Vgs2 and Vg1 must be tuned on a per circuit basis due to inherent process variations.

[0027] In contrast, embodiments of the disclosure have follower networks with referenced feedback that result in a lower input impedance for the bias circuit which maintains quiescent voltages at Vg1 and Vg2 as the RF amplifier transitions between small signal and large signal operation. Diodes can be substituted by resistors if applications desire higher input impedances.

[0028] The example embodiment of FIG. 4 is described more fully below. In example embodiments, the current mirrors 420, 422 positive non-inverting current mirrors. The first mirror comprises a first transistor FET Q1 420 where the drain is fed by the first reference current Iref 423. It is noted that FET Q1 420 is placed into saturation so that the current between its source (S) and drain (D) is substantially constant with variations in the voltage across its source (S) and drain (D). The first transistor 406 of the amplifier is provided having its gate electrode (G) connected to the gate electrode of FET Q1 420 via reference transistor 424, where the current through the drain electrode of transistor FET 406, can be regulated by the value Iref 423. It is noted that FET 420 and FET 406 are both placed into saturation.

[0029] The drain electrode (D) of FET 420 is coupled to its gate electrode (G) through a follower network comprised of FET 427 and a network comprising one or more serially coupled diodes, here, for example, diode Dn1, and another serial element, here a transistor load 429, as shown. The source electrode of FET 420 is coupled to ground potential, as indicated. The gate electrode (G) of FET 420, with the follower network produces an output which is fed to the gate electrode of the depletion-mode FET 406 of the RF amplifier. The gate electrode (G) of FET 406 is fed an input RF signal RFin. It is noted that since the FET 406 is here a depletion-mode FET, its gate electrode (G) is typically DC biased at a potential more negative than ground potential.

[0030] In the illustrated embodiment, FET 420 has its drain electrode (D) coupled to the first current reference 423 and to the gate electrode (G) of FET 427, as shown. The drain electrode (D) of FET 427 is also connected to Vdd2. The gate electrode (G) of FET 420 is connected to the source electrode (S) of FET 427 through diode Dn1, as shown. The source electrode (S) of FET 420 is connected to ground. The gate electrode (G) of FET 420 is also connected to the gate electrode (G) of amplifier FET 406 through an RF blocking inductor L1 and to Vss1 through FET 429a connected as a current source load resistance, as shown.

[0031] The gate electrode (G) of FET 406 is also RF coupled to an RF input signal, RFin through a DC blocking capacitor C1, as indicated. The source electrode (S) of FET 406 is connected to ground. The drain electrode (D) of FET 406 is coupled to the source(S) of amplifier FET 408 in a stacked arrangement. An RF blocking inductor L2 is coupled to the output RFout through DC blocking capacitor C2, as indicated. In the illustrated embodiment, the FETs are depletion-mode FETs (D-FETS).

[0032] The second mirror includes FET 422 coupled to the gate (G) of FET 431 in a follower configuration with series-coupled diodes Dn2. The gate electrode (G) of FET 422 is also connected to the gate electrode (G) of amplifier FET 408 through an RF blocking inductor L2 and to Vss2 through FET 429b connected as a current source load resistance, as shown The drain (D) of FET 422 is coupled to the second current source 426. The source (S) of FET 422 is connected to the drain (D) of FET 424. FET 424 source potential (S) is ground reference and its gate potential is set by the first current mirror. The output of the second mirror provides the bias signal Vg2 412 to the second transistor 408 in the stacked FET amplifier. As noted above, the first and second current mirrors are referenced to each other via the reference FET 424.

[0033] It is understood that that Vss1 and Vss2 can be at the same or different reference potentials. A reference potential may be less than ground reference or one may be less than ground reference and the other may be a more positive potential, ie., ground. It is further understood that any practical number of diodes can be used and that resistors can be used stead of, or in combination with, diodes in a follower configuration. It is further understood that any practical number of transistors can be used in stacked amplifier embodiments with respective control loops.

[0034] Having described exemplary embodiments of the disclosure, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0035] Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.