Two Active Loads for Differential Amplifiers: The Lee Load and the Current Mirror Load

1 6.012 - Microelectronic Devices and Circuits - 11/19/09 Two Active Loads for Differential Amplifiers: The Lee Load and the Current Mirror Load Two Active Loads for Differential Amplifiers: 1. Current mirror load 2. Lee load 1. The current mirror load: The current mirror load provides double-ended to single-ended conversion without suffering the loss of a factor of two in differential-mode gain (the common-mode gain is twice as large also, but still very small) . It comes in a variety of versions (pnp, npn, nMOS, p-MOS); the examples below use p-channel MOSFETs in the mirror loading an nchannel common-source differential gain stage. We can begin to understand how the current mirror load functions by referring to the two figures below. The active nature of the load doubles the current delivered to the load with differential-mode inputs, and while not sending any current to the load with common-mode inputs. The results of a more detailed analysis are listed below the figures. Difference-mode inputs: Common-mode inputs: V+ Q1 id V+ Q2 Q1 Q2 id ! 2 id id + vid/2 - id Q3 Q4 RL + - vid/2 - + vout + vic - ic ic ic ic Q3 Q4 IBIAS rob V- !0 RL + vout - + vic - IBIAS rob V- A full analysis shows that with differential inputs, ± vid/2, as on the left above, the output, vout, is given as: vout ≈ [2gm3/(go2 + go4 + GL)] vid/2 and with common-mode inputs, vic, as on the right above, the output, vout, is given by: vout ≈ [gob/2(gm2 + go4 + GL)] vic ≈ [gob/2 gm2] vic The total single-ended output for a pair of inputs to the stage is: vout ≈ [2gm3/(go2 + go4 + GL)] (vi1 – vi2)/2 + [gob/2 gm2] (vi1 + vi2)/2 6.012 - Microelectronic Devices and Circuits - 11/19/09 Two Active Loads for Differential Amplifiers: The Lee Load and the Current Mirror Load 2 2. The Lee load The Lee active load provides a typically high differential-mode gain and an unusually small common-mode gain. The conventional differential amplifier with a current-source load will have a common-mode gain of order unity, whereas the Lee Load yields a common-mode gain one to two orders of magnitude smaller [as much as something on the order of (VGS - VT)min/VA times smaller]. The Lee Load as normally drawn is shown below, but it is somewhat easier to V+ Q1 Q2 + vOUT1 Q5 + vIN1 - Q3 Q4 RL RL + vOUT2 Q6 + vIN2 - IBIAS rob V- understand if it is redrawn as shown below. In this presentation it is easier to see how the two outputs are fed back to the inputs of the active loads so as to cancel each other out in difference-mode, and to re-enforce each other in common-mode. V+ Q1 + vOUT1 - + vIN1 - Q3 Q2 + vOUT2 + vOUT1 Q5 RL + vOUT1 RL + vOUT2 Q6 IBIAS rob V- Q4 + vOUT2 - + vIN2 - 3 6.012 - Microelectronic Devices and Circuits - 11/19/09 Two Active Loads for Differential Amplifiers: The Lee Load and the Current Mirror Load To analyze quantitatively the Lee Load performance, we can do small signal analyses using half-circuit techniques and difference-mode and common-mode inputs. We first consider difference-mode inputs, and then common-mode: Difference-mode analysis: The circuit drawn with difference-mode inputs is shown below: V+ Q1 Q3 + vod/2 - + -vod/2 + vod/2 - + vid/2 - Q2 RL + vod/2 RL Q5 Q4 + -vod/2 - + -vod/2 Q6 + -vid/2 - IBIAS rob V- The linear equivalent half-circuit of this amplifier with difference-mode inputs is as illustrated below. Note that the gmvgs generators for the p-channel MOSFETs are in parallel with vod/2, and vgs is + or - vod/2, so the generators look like conductances of value + or - gm, respectively. + vid/2 + gm5vid/2 go5 go1 gm1 -gm3 (= -g m1) - go3 (= go1) GL vod/2 - Referring to the linear equivalent half-circuit, we see that vod/2 = [gm5/(go5 + go1 + go3 + gm1 - gm3 + GL)] vid/2 = [gm5/(go5 + 2 go1 + GL)] vid/2 Thus the difference-mode voltage gain, Avd is Avd = gm5/(go5 + 2 go1 + GL) We see that the gm factors in the denominator, which are the biggest factors there, are of equal magnitude, but of opposite sign, and thus cancel, resulting in a large differencemode gain. 4 6.012 - Microelectronic Devices and Circuits - 11/19/09 Two Active Loads for Differential Amplifiers: The Lee Load and the Current Mirror Load Common-mode analysis: The circuit drawn with common-mode inputs is shown below: V+ Q1 Q3 + voc - Q2 + voc + voc - + voc RL RL + voc - + voc - Q5 + vic - Q4 Q6 + vic - IBIAS rob V- The linear equivalent half-circuit of this amplifier with common-mode inputs is as illustrated below: + + + gm5vgs5 vgs5 vic go5 go1 - gm1 gm3 (= g m1) go3 (= go1) GL voc gob/2 - - Referring to the linear equivalent half-circuit, we see that voc ≈ [gob/2(go1 + go3 + gm1 + gm3 + GL)] vic It is clear that this time the gm terms are not going to cancel. Continuing we find that they in fact dominate the denominator: voc ≈ [gob/2(2go1 + 2 gm1 + GL)] vic ≈ [gob/4 gm1] vic Thus the common-mode voltage gain, Avc is approximately Avc ≈ gob/4gm1 Avc is typically quite small because gob will be very small, and gm1 will be much larger.