That output should be obtainable, but how do you know it is saturating? Maybe the current mirrors can not handle it?
When you have a problem like this you have to go through each stage one by one and find out what stage is causing it.
On a separate plug board, try testing the 741 to see that it can get up to 10v peak on the output with a gain of 10 and 1v peak on the input. It's easy enough to wire this up with a couple resistors. Test that and see what results you get. You might also test all the 741 packages you have to make sure they all work ok.
Another problem caused by a solderless breadboard.Breadboarding the circuit may well account for unexpected gain values. I just ran a sim with a stray 10 mΩ connected between the bases of the two transistors of a current mirror. The result was a ~50 dB increase of the common mode gain.
1) Your transistors are not matched.I'm still thinking about what makes the output of current mirrors to have 0.5v while 1v is applied to one of first buffers and the other is grounded. It's very strange why can't current mirrors handle changes in resistors value?
Those transistor arrays are obsolete and have not been made for many years. Their datasheet does not say the transistors are matched and does not show matching spec's. The datasheet says that the darlington arrays have four separate chips. The datasheet does not say if the transistor arrays use a single chip or uses four chips.This circuit has 4 npn transistors and 4 pnp transistors. I used one transistor array (Tpq2907) for pnp transistors and one transistor array (Mpq2222) for pnp transistors. So pnp transistors and npn transistors are matched.
Hi,
You have a problem, so something is wrong. When something is wrong it doesnt always obey basic theory because there is a fact or set of facts missing from all the arguments. To fill in the facts, you MUST make measurements.
For example, there could be something wrong with one or more of the transistors in either IC. If that was the case then you would see a change in the current through the current mirror output(s).
If there is something wrong with the output of the op amp, then test the op amp. Test it where it stands if you use a plugboard.
As others have noted, when dealing with circuits that depend highly on exact voltages (current mirrors, op amps) a plugboard could mess everything up because there will be unknown small voltage drops in various places. I ran into this problem too when trying to calibrate a digital volt meter that was assembled on a solderless breadboard (plugboard). Even moving the breadboard around a little changed the reading because the wires would move ever so slightly, and that changed the voltages drops in various places in the circuit. It's impossible to do it on a plugboard because of this. The readings could easily change by 10mv, which is far too much for a digital volt meter that has to read 5 volts with 4 decimal place accuracy (like 4.675 volts for example). It's also too much for a current mirror, and can cause large offsets with op amps. Even thermal DC bias on some IC pins causes problems, even with excellent connections, because the offset is generated right in the wires, and it is DC not AC so that means we get an additional unwanted offset.
As a test of the op amp input buffer, you can ground the center of the two 10k that are in series. See what happens.
You should really try to start to think about how you can resolve these little problems yourself via making measurements. Think about what could go wrong, then make some measurements.
If the op amp has an offset, it is caused by something. Make measurements until you find something that does not agree with theory, then investigate.
Watch for intermittent results in the measurements which could indicate a connection is bad somewhere.
Jiggle the wires around a little, see what changes in the measurements. Track it down to one wire or several wires. Build it on a real breadboard (soldered) if needed.
You can also use a PC board with DIP patterns with sockets mounted on the board. You can plug in the IC's that way, and then solder leads to the PC board itself, making jumpers where needed. This isnt optimum either, but it's better than a solderless breadboard.
The board is made with socket holes for the IC sockets, and breakout clad patterns for the IC's that break out the pins to individual solder pads. You can then solder to the solder pads using wires or components to make all the connections. With this arrangement when you move the board around not much changes, and the connections are better.
The current package being used surely does not contain matched transistors. What the effects will be is more DC offset, but there may be a simpler problem with the op amp output so that should still be investigated.
About the mismatched transistors...
You have to buy transistors that say they are matched on the data sheet. Take a look at MAT14 for example made by Analog Devices.
But there could still be another problem other than that.
About the spice programs...
In this way we can get familiar with the circuit in a way that would be much harder without the spice program. Think a resistor value that is off by 1 percent is messing things up? Change it in the spice circuit and see if you get the same results with the simulation as you did with the messed up measurement in real life. Want to know how the mismatch of transistor emitter base junctions affects the output? Add a tiny battery of 1 to 10 percent of the voltage of the typical base emitter voltage in series with the base then ask the question: "How much does this change my circuit output?". If the answer is: "A lot" then you might be in trouble with the circuit, but if the answer is: "Not too much" then you might be ok with it.
Eh? Mismatching the input resistors (10k) by 10% results in a ~100 dB change in the common mode gain at low frequencies !!Resistor value mismatch doesn't change the simulation result.
What do you mean? what problem?
Even that will certainly change the CM gain. LTspice shows a CM gain increase at 100Hz of ~85 dB when the input resistors are mismatched by only 0.1%. In real life, matching all discrete gain-affecting components to better than 0.1% will be very difficult.I changed them by 1% not 10.
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