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14th February 2010, 12:13 #1
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Low output impedance via MOSFET?
I am supposed to design a power amplifier which has a MOSFET output stage. It should deliver as much power as possible to a 20 ohm load. Therefore, I am trying to make the output impedance as near to 20 ohm as possible.
Thus, obviously I should use a common drain MOSFET. However, the problem is output impedance of a common drain mosfet is approximately 1/gm with gm=2sqrt(KnId). With standart values of Kn we require Id in the order of Amperes to obtain 1/gm as 20 ohm. Typicially I can obtain around 12kOhm output impedance with this configuration. So, how can I solve this problem, ie how can I make the output impedance near 20 ohm using a MOSFET?
thanks in advance :)

14th February 2010, 12:13

14th February 2010, 13:31 #2
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Re: Low output impedance via MOSFET?
Originally Posted by oahsen
Originally Posted by oahsen
1 members found this post helpful.

14th February 2010, 15:12 #3
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Re: Low output impedance via MOSFET?
Originally Posted by erikl
For id=un*cox*W/L*(VgsVtn)^2 where in the datasheet un,cox and W/L is written?

14th February 2010, 15:12

14th February 2010, 15:41 #4
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Re: Low output impedance via MOSFET?
Originally Posted by oahsen
BTW: With an nMOSFET in common source configuration you can only control the negative path (low side) of the power supply. You could, however, use an nMOS in common drain configuration (i.e. as source follower) to control the positive path (high side), but then it won't be an LDO (Low Drop Out) controller. For an LDO controlling the high side, you'd need a pMOS.

14th February 2010, 15:50 #5
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Low output impedance via MOSFET?
You didn't yet tell a word about current range and signal frequencies.

14th February 2010, 15:50

15th February 2010, 17:42 #6
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Re: Low output impedance via MOSFET?
Originally Posted by erikl
Code:.SUBCKT FDP8880 2 1 3 Ca 12 8 9.5e10 Cb 15 14 9.5e10 Cin 6 8 1.15e9 Dbody 7 5 DbodyMOD Dbreak 5 11 DbreakMOD Dplcap 10 5 DplcapMOD Ebreak 11 7 17 18 32.88 Eds 14 8 5 8 1 Egs 13 8 6 8 1 Esg 6 10 6 8 1 Evthres 6 21 19 8 1 Evtemp 20 6 18 22 1 It 8 17 1 Lgate 1 9 5.3e9 Ldrain 2 5 1.0e9 Lsource 3 7 1.7e9 RLgate 1 9 53 RLdrain 2 5 10 RLsource 3 7 17 Mmed 16 6 8 8 MmedMOD Mstro 16 6 8 8 MstroMOD Mweak 16 21 8 8 MweakMOD Rbreak 17 18 RbreakMOD 1 Rdrain 50 16 RdrainMOD 1.0e3 Rgate 9 20 2.2 RSLC1 5 51 RSLCMOD 1e6 RSLC2 5 50 1e3 Rsource 8 7 RsourceMOD 6.8e3 Rvthres 22 8 RvthresMOD 1 Rvtemp 18 19 RvtempMOD 1 S1a 6 12 13 8 S1AMOD S1b 13 12 13 8 S1BMOD S2a 6 15 14 13 S2AMOD S2b 13 15 14 13 S2BMOD Vbat 22 19 DC 1 ESLC 51 50 VALUE={(V(5,51)/ABS(V(5,51)))*(PWR(V(5,51)/(1e6*170),5))} .MODEL DbodyMOD D (IS=3E12 IKF=10 N=1.01 RS=5e3 TRS1=8e4 TRS2=2e7 + CJO=4.8e10 M=0.55 TT=1e11 XTI=2) .MODEL DbreakMOD D (RS=0.2 TRS1=1e3 TRS2=8.8e6) .MODEL DplcapMOD D (CJO=5.5e10 IS=1e30 N=10 M=0.45) .MODEL MstroMOD NMOS (VTO=2.10 KP=170 IS=1e30 N=10 TOX=1 L=1u W=1u) .MODEL MmedMOD NMOS (VTO=1.75 KP=10 IS=1e30 N=10 TOX=1 L=1u W=1u RG=2.2) .MODEL MweakMOD NMOS (VTO=1.39 KP=0.05 IS=1e30 N=10 TOX=1 L=1u W=1u RG=22 RS=0.1) .MODEL RbreakMOD RES (TC1=8.0e4 TC2=8e7) .MODEL RdrainMOD RES (TC1=12e3 TC2=.35e4) .MODEL RSLCMOD RES (TC1=9e4 TC2=1e6) .MODEL RsourceMOD RES (TC1=5e3 TC2=1e6) .MODEL RvtempMOD RES (TC1=2.78e3 TC2=1.5e6) .MODEL RvthresMOD RES (TC1=1e3 TC2=8.2e6) MODEL S1AMOD VSWITCH (RON=1e5 ROFF=0.1 VON=4 VOFF=3.5) .MODEL S1BMOD VSWITCH (RON=1e5 ROFF=0.1 VON=3.5 VOFF=4) .MODEL S2AMOD VSWITCH (RON=1e5 ROFF=0.1 VON=1.3 VOFF=0.8) .MODEL S2BMOD VSWITCH (RON=1e5 ROFF=0.1 VON=0.8 VOFF=1.3) .ENDS
Added after 3 minutes:
Originally Posted by FvM

15th February 2010, 22:41 #7
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Re: Low output impedance via MOSFET?
A single transistor means class A operation. If the load is resistive, the DC bias current causes a dissipation twice the signal power
(assuming a sine signal). Is this what you intend? Otherwise, you should think about a pushpull amplifier.

16th February 2010, 00:18 #8
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Re: Low output impedance via MOSFET?
Originally Posted by FvM
Any suggestions on this problem?
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