4 DC-OFFSETs

[dr.power]

Hi guys,

I lust want to inject A variable DC-Offset to the inputs of 4 chips in a circuit. The level of the DC-Offset's has to be the same for all inputs but just inverted by 180 degrees for two of the input of the said chips.
So I seem have to use a pot and change it to get for instance say "2" "-2" "2" "-2" volts or other similar in level voltages.
Unfortunately I am not able to connect all inputs of the chips directly to the said pot because each input is driven by a separate ac signal too.

Any idea how to do so?

Thanks

 

[dr.power]

No idea Plz?

 

[audioguru]

It is easy to change the DC offset voltage of a capacitive-coupled (DC gain is 1) inverting opamp because the amount of DC fed to the non-inverting input is the same as the output DC offset voltage.
But a non-inverting opamp is a little more difficult.

 

[crutschow]

You could AC couple the AC inputs using series capacitors. Then you can inject the DC on the output side of the capacitors. The capacitor must be large enough to pass the lowest AC frequency of interest.

You can get the inverted DC voltages by using an op amp connected as an inverting amp with a gain of one.

 

[Grossel]

I have an idea.

All resistors have equal resistance

[edit]
Had to color it. Red represent non-inverting input.

 

[dr.power]

Thanks friends for all your inputs...

Actually I want to use just ONE pot for the job, makes sense? and thats my main problem how to perform that?

 

[Grossel]

Thanks friends for all your inputs...

Actually I want to use just ONE pot for the job, makes sense? and thats my main problem how to perform that?

Use the pot as a basic voltage divider and connedct it to "Input DC voltage".

 

[dr.power]

Use the pot as a basic voltage divider and connedct it to "Input DC voltage".

Grossel, Thanks for your input,

Please tale another look at my first post..
I want to inject the same DC offset to 4 inputs through ONE pot while each input carries an independent AC signal too, So I do not like to connect all inputs directly to one pot.

 

[Grossel]

Then you can use four separate opamp adder circuits.

 

[MrAl]

Hi guys,

I lust want to inject A variable DC-Offset to the inputs of 4 chips in a circuit. The level of the DC-Offset's has to be the same for all inputs but just inverted by 180 degrees for two of the input of the said chips.
So I seem have to use a pot and change it to get for instance say "2" "-2" "2" "-2" volts or other similar in level voltages.
Unfortunately I am not able to connect all inputs of the chips directly to the said pot because each input is driven by a separate ac signal too.

Any idea how to do so?

Thanks

Hi,

Here's a diagram of one way to do this.

Connect your adjustment pot connected as a positive voltage divider to the input of that upper op amp.

Connect capacitors to the junctions of the resistors to couple your AC signal in. The junction of the resistors will be your output too.

Shown is one positive type circuit and one negative type circuit. To get the other circuits, connect more resistors to the op amp outputs.

The matching of the input/outputs are related to the precision of the resistors, but if you need really good accuracy on the DC setting you could make each lower (or upper, or both) resistor a pot itself too, and adjust it once to sync all the outputs. Once done, you only have to adjust the input pot to change all of the settings.

 

[dr.power]

Then you can use four separate opamp adder circuits.

Four Adder?!!!!

 

[dr.power]

Hi,

Here's a diagram of one way to do this.

Thanks MrAl for your assistance, You are indeed very helpful.

Connect your adjustment pot connected as a positive voltage divider to the input of that upper op amp.

What do you mean by the POSITIVE voltage divider? Are you mean that I have to use one of the side pins of the pot to the GROUND instead of the Negative voltage so that do not let the Pot voltage to reach to the negative voltages?

Connect capacitors to the junctions of the resistors to couple your AC signal in. The junction of the resistors will be your output too.

Actually I do not know why you decided to use a resistor divider at the output of the op amps?? Honestly I was thinking of a similar circuit as your pic here in this post before starting this thread but by using 4 OP-Amps One as non inverting having 2 the same resistors as the Rin and Rf and then use an inverting OP-amps connected just like the bottom OP-amps of your post But do not use any divider at the output, Then use another pair the same as I tol....

Shown is one positive type circuit and one negative type circuit. To get the other circuits, connect more resistors to the op amp outputs.

Sorry I can not understand your meaning just right. Are you meaning that I do not need another pair of op-amps?!

The matching of the input/outputs are related to the precision of the resistors, but if you need really good accuracy on the DC setting you could make each lower (or upper, or both) resistor a pot itself too, and adjust it once to sync all the outputs. Once done, you only have to adjust the input pot to change all of the settings.

I just want the same voltage to create the carrier level of a SSB circuit, It mean that I need almost accurate DC-Offsets...

 

[dr.power]

Here's the circuit which I am talking about:
I want the DC-OFFSET which I am talking about for R S T U, in the pic, So I can remove the related POTS and just use one to control the level of the DC-OFFSET's so that Inject and change the magnitude of the carrier for the circuit...

 

[dr.power]

Ok, I meant some thing like the below pic MrAl,

Please consider that the circuit is in the DSB-WC mode yet, I have opened the connections which make the circuit as a SSB generator for simplicity till add the proper DC-offset.

Please take a look at the DC-Offset section and see if It needs any modification?
Honestly I prefer to do it by just 2 op-amos instead of 4, but I do not have any idea about accuracy of doing so...

Honestly I do not really know how to calculate the proper values of R26, R30, R34 and R48 which are going to the input of the modulator so that give it some DC=offset to produce the carrier more the modulator?

P.S Nigel, please tak a look at the attached Pic, It indicates that my idea regarding to have a Modulator based on CHOPPER keys does work just nice...

 

[dr.power]

Another pic showing Where the DC-Offsets are connected to....

 

[MrAl]

Hello again,


By "positive voltage divider" i mean the pot only connects to the positive supply and the ground.

It looks like your circuit has to 'shift' the DC output of two of the outputs when you switch modes. This means we'll need to modify the circuit a little. You need DC levels for RSTU right?

 

[dr.power]

Hello again,


By "positive voltage divider" i mean the pot only connects to the positive supply and the ground.

Thanks for your input,
Yea I Did so as you can see in my pics.

It looks like your circuit has to 'shift' the DC output of two of the outputs when you switch modes. This means we'll need to modify the circuit a little. You need DC levels for RSTU right?

[/QUOTE]

What do you mean by "RSTU"?
As you can see in the Pics of my above posts, The DC-OFFSET is going to the input of the modulator, of course the said dc offset is ADDED or summed by the modulating audio (message signal), So that creates some dc offset to let the carrier to comes out of the modulator too. If you take a look at the formula for AM modulation you will notice that there is a constant value summed by the message, this constant value is the DC-OFFSET which I a trying to add to the input of the modulator...

 

[dr.power]

P course You can simply do not inject the dc offset to the circuit so that get DSB-cC or SSB-SC on the output of the modulator..

 

[MrAl]

Hello again,


I took a look at your other pic, and it appears that you are not going to use the SSB switch any longer, or are you?
If you intend to be able to switch as shown by the SSB/AM switch then you need to be able to use two different settings for the pots, not just one setting as your other diagram shows.

RSTU is of course the letters on the schematic. You shouldnt need to change those resistors if it was working as it is now.

 

[dr.power]

I took a look at your other pic, and it appears that you are not going to use the SSB switch any longer, or are you?

Actually I guess that I am able to get SSB-SC without that switch too, Just by lowering the pot to its minimum (i.e ground).

If you intend to be able to switch as shown by the SSB/AM switch then you need to be able to use two different settings for the pots, not just one setting as your other diagram shows.

Indeed I am not able to see why the original designer has used 2 sets of pots?!
I think just one pot will be able to produce both the SSB-WC and SSB-SC.
Are you thinking that the designer has put the said SSB/AM to produce ether the ssb and "DSB"? If so, I am wondering how it does so?!

RSTU is of course the letters on the schematic. You shouldnt need to change those resistors if it was working as it is now.

Oh yea,

SO to response to your past question regarding RSTU, I should say, Yes I want to have SSB-WC at the output, it means that I need the dc offset for RSTU.