Bias Resistor for AC Coupled Voltage Follower

Thread Starter

phani250

Joined Nov 27, 2011
6
DC coupled voltage follower doesn't need the bias resistor to provide the ground path for bias current, but AC coupled voltage follower needs the bias resistor to provide the ground path for bias current. AC coupled voltage follower has a series capacitor with input voltage to block the dc signal.Can someone please explain what is preventing the AC coupled voltage follower for not having the ground path for the bias current when there is no bias resistor?
 

crutschow

Joined Mar 14, 2008
34,280
Can someone please explain what is preventing the AC coupled voltage follower for not having the ground path for the bias current when there is no bias resistor?
Capacitors cannot conduct DC so with AC coupling you need the resistor for the DC bias current, otherwise it has no place to go.
 

#12

Joined Nov 30, 2010
18,224
Capacitors cannot conduct DC so with AC coupling you need the resistor for the DC bias current, otherwise it has no place to go.
That gets straight to the point. No matter what circuit gets posted, there must be a DC path somewhere.
 

MrAl

Joined Jun 17, 2014
11,389
DC coupled voltage follower doesn't need the bias resistor to provide the ground path for bias current, but AC coupled voltage follower needs the bias resistor to provide the ground path for bias current. AC coupled voltage follower has a series capacitor with input voltage to block the dc signal.Can someone please explain what is preventing the AC coupled voltage follower for not having the ground path for the bias current when there is no bias resistor?
Hi,

Would be nice to see the circuit here :)
 

Thread Starter

phani250

Joined Nov 27, 2011
6
Thanks all, I understand that it needs a path for the dc bias current to flow. But the bias currents are always dc in nature?
 

#12

Joined Nov 30, 2010
18,224
Even if you check for, "Chopper Stabilized" op-amps, the type of input transistor does not change from N-type to P-type. The current just gets turned on and off, so the bias current always flows the same way. Blocking that current, however small it is, with a capacitor, will quickly stop the amplifier from working.
 

ian field

Joined Oct 27, 2012
6,536
DC coupled voltage follower doesn't need the bias resistor to provide the ground path for bias current, but AC coupled voltage follower needs the bias resistor to provide the ground path for bias current. AC coupled voltage follower has a series capacitor with input voltage to block the dc signal.Can someone please explain what is preventing the AC coupled voltage follower for not having the ground path for the bias current when there is no bias resistor?
Usually; you'd bias the voltage follower with a potential divider across the supply rails.

You can calculate the ratio of the 2 resistors for any given fraction of the supply voltage - 50/50 is a good starting point as it should (in most cases) give you maximum output voltage swing, and symmetrical clipping, if you push it that far.
 

ian field

Joined Oct 27, 2012
6,536
Transistor? Op-amp? Please post a circuit drawing.
Its not like it actually makes much difference.

True; an OP-AMP should have higher input resistance - but the basic principles apply just as much to an emitter follower.

Just don't forget to allow for Ib.
 
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