I use 3 diode connected active load amplifier shown before as low gain stage.I use three of them to build the total amplifier.It's power consume is
about 60uw for one stage.I also use another gain variable amplifier to make the whole amplifier to vary the gain.And the output stage which was shown in Figure 3(the output stage).The reasons are described as below:
(1)for such condition Maximum Gain Variation *( 0 < f < 20MHz, 0oC < T < 80oC) < +/- 1 dB ,I have to let the output impedance small so that the amplifier can meet it.As we all know,Usually W3db=1/(2*pi*Rout Cload),in
order to improve W3db,Rout have to be reduced.So output stage such as cascode folder structure cann't use ,gain boost technology also cann't meet the design.connected active load amplifier can meet high w3db and unity gain frequency,though it's gain is smaller.Am I right?but there are also some problems. Because multiple stage amplifer, I have to make every stage stable,that's means phase margin should large than 60 which is required in design.The main problem maybe in the output stage.As I am described before, my output stage still can not meet the design.Such output stage the output resistance =1/gm=(vgs-vth)/2Id, for w3db=1/(2*pi*Rout Cload)>20M,cload=1pf, Routshoud<7.9577k,it just for 3db.for 1 db, rout=5k.if r=5k, vgs-vth=0.5v(Becuse the dc output should equal=0).Id>=50u, idtotal=100ua,the power should larger than 200uw.According to my simulation let Id=50u,still can't meet w1db=20M,If I improve the power I can meet but It will large than 500uw.Using such output stage the swing still can't meet the design.
Am I right.Thanks