no amplifying signal from opamp

Thread Starter

Tri Nugroho

Joined Jul 19, 2015
4
hi all, I am in trouble making an analog signal amplifier using a configuration from someone's previous post. here is the carrier signal split where positive and negative signal shifted 180 degree then i check the output of both opamp and there is no problem but if the opamp output is put in to the mosfet there is no gain at the mosfet output. is there anything I miss?
 

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ScottWang

Joined Aug 23, 2012
6,963
R5 decreasing to 33K or R6 increasing to 330K.
If it still doesn't works then exchange the input(-) and (+) of two op amp and try again.
 

Alec_t

Joined Sep 17, 2013
11,740
The gate voltage isn't high enough to turn on the FET. It needs Vgs=10V to turn on fully, and as much as 4V just to turn on enough to pass a few uA.
 

Thread Starter

Tri Nugroho

Joined Jul 19, 2015
4
R5 decreasing to 33K or R6 increasing to 330K.
If it still doesn't works then exchange the input(-) and (+) of two op amp and try again.
i change the resistor value the output both opamp become unbalance, i repeat it for acceptable scope signal but dead zone increase between channel A & B
 

Thread Starter

Tri Nugroho

Joined Jul 19, 2015
4
The gate voltage isn't high enough to turn on the FET. It needs Vgs=10V to turn on fully, and as much as 4V just to turn on enough to pass a few uA.
i have supply up to 30v +/- should i rise the supply peak voltage? i know Vgs gate need 10V minimum the sine signal above is actualy amplitude modulation i want to drive Vgs rise from 5V to 10V then back down, would that shaping Vds also?
 

Thread Starter

Tri Nugroho

Joined Jul 19, 2015
4
If Vgs(thr) is <5V the FET would be permanently turned on. Your FET stage is not configured as a linear amplifier, if that is what you want.
yes exactly, after read irfz44n datasheet there is no possibility to shaping VDS even low pass filter applied. im trying to figure it out how to drive AM transmitter by split single sine signal, both opamp are has nice balanced positive and negative swing with no dead zone between them. do you have suggestion?
 
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