Can we use one op-amp to achieve the non-inverting V sum and buffer for high Iout

Hi All, I am designing a circuit to achieve the non-inverting sum and current buffer. We can use two op-amps for each process, like this picture.



I am wondering is it possible to merge them together, by just one op-amp to achieve the same goal? No high precision is required in my application. Thank you!

Hi,

You may connect the input resistors directely to Opamp2.
Maybe you need an additional resistor to GND

(I don't think you need the gain, that the first Opamp generates....but only you know the specifications)

Klaus

KlausST said:

Hi,

You may connect the input resistors directely to Opamp2.
Maybe you need an additional resistor to GND

(I don't think you need the gain, that the first Opamp generates....but only you know the specifications)

Klaus

Click to expand...

Hi Klaus, is this what you are talking about? Thank you!
All resistors can be tuned to generate the target output.

Hi, can anyone give me some suggestion how to choose a discrete FET for my application? If the Vin and VCC are both 5.5V, and Vout can go up to 5V, I guess a BJT cannot be used.
My questions are:
(1) if I want to choose a MOSFET to operate in its triode region (current capability around 50mA), should I use PMOS or NMOS?
(2) I found most of the discrete FETs are used as switches. Can these products also be used in their triode region as the variable resistors?

Hi,

We can not know the values of your inputs, you have to give them...

Therefore I only can assume that you rather should connect the GND-resistor to IN+.
For calculation we need your values..

Mosfet: voltage as you need plus some overhead, the same is with current, power: worst case = U x I, V_gs_th < 2.5V

Klaus

KlausST said:

Hi,

We can not know the values of your inputs, you have to give them...

Therefore I only can assume that you rather should connect the GND-resistor to IN+.
For calculation we need your values..

Mosfet: voltage as you need plus some overhead, the same is with current, power: worst case = U x I, V_gs_th < 2.5V

Klaus

Click to expand...

Thank you Klaus. Sorry for the confusion. My V1 is around 1.2V and V2 is 3.2V. My target is to achieve a Vout=5V.
(1) Sorry I do not understand your advice "rather should connect the GND-resistor to IN+". Would you give me a picture of the connection?
(2) Can I use a PNP device like MPSA92 in the current buffer? I am using the general opamp in LTC1541.

Hi,

My V1 is around 1.2V and V2 is 3.2V. My target is to achieve a Vout=5V.

Click to expand...

You build a current source... what do you mean with 5V? 5V is no current.

****

1) first you need to tell the relationship between input voltage and output current.

******

2) we don't know what circuit you need. With a NPN you create a low side current source. With PNP and some changes you can build a high side current source.

Klaus

KlausST said:

Hi,


You build a current source... what do you mean with 5V? 5V is no current.

****

1) first you need to tell the relationship between input voltage and output current.

******

2) we don't know what circuit you need. With a NPN you create a low side current source. With PNP and some changes you can build a high side current source.

Klaus

Click to expand...

Thank you Klaus. Actually I am trying to use discrete opamp and PNP or NPN to build a regulator, such as an LDO. The load of the circuit is just a pure resistor. The only difference between this circuit and an conventional LDO is that its Vref should be a sum of V1 and V2, instead of a 1.2V from a reference circuit. If we assume the current capability is within the limit for the PNP/NPN, would you teach me how to build this regulator?

Hi,

Sorry, my mistake.
Your circuit shows a voltage regualtor. Use the bjt circuit of post#4
So indeed it can work.

But you need to specify the characteristic.
U_out = gain_x * (U1 + U2)

Then the feedback resistors have to be calcualted like for an non-inverting opamp circuit, but with doubled gain: (gain_n = 2 * gain_x.)

*****

OPAMP:
* You need to select an OPAMP which is capable for common_mode_input_voltage_range: 0...(V1 + V2)/2
* V_out (and supply) for an output_voltage_range of 0.5V ... V_load + 1V

****
I recommend to use a simulation tool. There are many free tools around.

Klaus