Latch circuit, positive feedback

[020170]

positive feedback latch

next picture is an example Latch circuit.

**broken link removed**

I know this circuit what is called that "Latch circuit".

but it is not simple. in last week, someone told me that this circuit had negative impedance. and then amp's Gain is unlimited.

I heard that negative impedance. but I don't understand exaclty.

why does call that nagative impedance circuit?

and this circuit had property that bandwidth growth and positive feedback. but I don't understand exaclty.

someone who can tell me in detail.

thanks.

 

[flatulent]

latch circuit

All flip flop circuits have positive feedback in the linear region. This allows the signal level to grow until it hits the power or ground levels.

 

[020170]

latch positive feedback

Could you tell me more information about positive feedback and negative impedance? I don't understand yet exactly. is there any book dealt with positive feedback and negative impedance in detail?

 

[flatulent]

small signal model latch

You need to go to the library and look at a transistor logic circuit textbook old enough to be from the pre-IC days.

 

[Btrend]

ic latching circuits

check this

 

[maxwellequ]

svenson latch positive feedback

If the NMOS differential transistors had a load constituted by two PMOS transistors connected has diodes (gate connected to the drain), then the output impedance would be 1/gmp (gmp-> transconductance of the PMOS transistors).

When the PMOS transistors have their gates cross-connected, the output impedance is -1/gmp. To prove this you just have to analyze the small signal model of this circuit.

 

[darkk]

call latch circuit

Simply use the simplest model to take place th top cross-coupled pmos transistors, add a reference voltage source and then you can solve the equation to find the current flow back to voltage source's plus end.

 

[laglead]

simple latch circuit

Hi, 020170,
Actually, there is no real "negative resistor". But some circuits have "negative resistance" property.
The general equation I=V/R means any increase of voltage (referred to the other point, e.g. ground) at one node can lead to increase of current in the direction from that node to ground. But in some cases the increase of current is in the direction from ground to that node. That is the "negative resistance".
Then above equation becomes I=V/(-R).
In your circuit, when Q2 gate voltage rises, its drain voltage also rises. But current of Q3 increases in the direction from power to Q2's drain. So, at that node we can see negative resistance.
Intuitively understanding, when voltage rise positive resistance "pull charge off" that node (fight the effect of voltage rise, negative feedback) while negative resistance "push charge into" that node ( boost the effect of voltage rise, positive feedback). So, negative resistance is always linked to positive feedback.

 

[montage3000]

positive feedback comparator latch

it means when voltage increase, current reduces

 

[laglead]

positive feedback analysis in latch

it means when voltage increase, current reduces

Yes. But we should take this "increase" or "reduce" into consideration with symbol of minus and plus. For example, "+2mA" is reduced to "-5mA". The + and - can be defined as into or out of one node.