# Circuit Idea/Group 57a
## About Us

Greetings to all circuit enthusiasts, dreamers and people who don't know anything about electrical engineering and stuff. If you are the 3d type, don't bother, because, as disciples of Mr. Mechkov (aka "circuit fantasist", aka the initiator of this page) we have learned that complex mathematics, formulas and confusing diagrams just make things worse (if that's even possible). So, what we are going to do is put our "learned" experience through the course in simple pictures with basic and logical explanations, with fewer numbers as possible. Cheers: The 57gr. team

## Lab 1 Building LED Voltage Indicator using transistor switches
### Whiteboard

Today's task, although may seem simple at first glance contains some interesting and cunning techniques about integrating digital and analogue circuit ideas together. The basic idea of the indicator is illustrated in this simple diagram below.
We start out with building bipolar power supply consisting of two 12V sources (which we'll also use in the further projects). We add the two resistors having in mind the semiconductor properties of each LED, depending on it's colour (green 2,1-2,2V; red 1,6-1,7V operating voltage). The next step will be to find effective, yet simple way to switch the current with ease between the LED branches. Here comes in handy an idea from ECL circuits, known as current steering. The current steering idea using simple switches, as shown on the white board is in the photo on the left.
The whole power block, including the LED's V-A characteristics and the ECL current steering principle is shown afterward.
The next step is to replace this simple switches with bipolar transistors. For our needs they should be different types, one PNP and the other NPN. Then we connect the transistor bases to each other and than to the input signal - thus the one will operate on the negative side of the signal (PNP) and the other, on the positive one (NPN). For input we'll use a variable resistor wired as a potentiometer. The middle point on the potentiometer is appearing to be 0 V, so it's called virtual ground. Our circuit is also going to be complementary - consisting of two opposite parts (negative and positive). When risen to levels above 0,6-0,7V the input voltage activates one of the BJTs (depending on the polarity) and there is current flowing through the collector-emitter junction. This activates the corresponding LED and makes the green go off because it's voltage has fallen to 1,7V.
Our next task will be to properly position the transistors and the two red LEDs. After trying various configurations, we managed to discover the optimal way shown on the whiteboard drawing of the final circuit. When there is no signal at the input of the BJT, the green LED is on thus receiving current from the voltage source and the two resistors. When risen to levels above 0,6-0,7V, the input voltage activates one of the BJTs (depending on the polarity) and there is current flowing through the collector-emitter junction. This activates the corresponding LED and makes the green go off because it's voltage has fallen to 1,7V.
In the two different scenarios, one of the transistors connects the corresponding red LED in parallel to the green one, while shortens the other red. Another benefit of this simple but powerful method is the big tolerance of the output and input voltages. It's also very flexible, because the zero indicator (the green LED) can be transformed to indicate various voltage levels just by adjusting the current-limiting resistors (R1, R2).

### Building the circuit

After understanding the final idea and structure of the circuit it's time to do it with real elements on the PCB.
See also 3-LED voltage indicator (an inventor's story)

### Resources

Virtual ground
Emitter-coupled logic
Revealing the Truth about ECL Circuits

## Lab 2 Investigating the common emitter BJT topology

Well, that's maybe one of the most familiar ways of connecting a BJT, known for its voltage amplifying capabilities. Today we'll take a deeper look in this famous circuit and investigate it's properties in practice. In fact the term 'amplifying' is a bit misleading, knowing the nature of the energy, so a more accurate way is to describe the process as 'regulating' energy. When we place such 'regulating' device, such as a transistor in the way of constant energy source it acts like a resistance which absorbs and converts energy in a way that makes this source no more constant but controlled. So we can refer to such devices as 'electric regulated resistors'.
We'll start with building the simplest circuit with n-p-n BJT transistor. The emitter will be tied to the ground and the base will act like a controlling input. We than can derive from this all the static characteristics and so on. For practical investigation we'll try to connect the BJT to a bipolar power supply. By doing this we again will observe the phenomenon 'virtual ground' in the middle. This resembles the so called 'push-pull output', but using just one transistor. As you can see in the pictures we used the DACs input for source and got some trouble following the base current route.

## Lab 3 Investigating the common-collector topology (aka emitter follower)

As we try to figure out the application of this rather odd looking circuit (to some it may look like the emitter is common, but in fact the collector is tied to the power supply rail) we stumble upon the idea of active repeating. To achieve a simple repeating of a voltage (or following) we may try to use a simple conducting wire. But soon we'll realize that this is not an option, because part of the voltage is lost due to the inner resistance of the conductor. So we'll have to find a different way of doing this. Luckily Mr. Mechkov has some inspiring examples in real life to share with us. Such examples we see everyday-in the driver controlling the cars speed, to the voice changing depending the surroundings. All examples we can think of have something in common-that's the active following. This is illustrated in the block diagram. Analyzing it we see that this is the principle of the negative feedback. In our case the comparing device will be played by the base-emitter junction and the regulating device will be the collector-emitter. The transistor will be than comparing the output emitter voltage with the input and matching both to be equal.

## Lab 4 Op-amp fundamentals, voltage follower and it's usage in Gyrator circuit

The op-amp (short for operational amplifier)is a key element of many useful circuits. As we all know, it possesses great amplifying capabilities-reaching easily 200 000 or more amp ratio. Have said that we can describe it's function as based on 'redundancy'. One of our tasks today will be to take this enormous ratio and scale it down to just 1(follower). This task may seem useless at first, as it was thought when the famous Harold Black made his discovery(the negative feedback), which later made a huge impact on modern engineering.
Lets begin our basic understanding of the op-amp. We see that it has two inputs-inverting and non-inverting. The two types are used depending our purpose-using negative or positive feedback. In addition to that we have also dual polar power supply. If we connect two 12V batteries and a load to make the picture complete we get the similar symmetric circuit we encountered before. It resembles a bridge circuit. The next step will be to finish the circuit with connecting two batteries to the differential inputs. The final variant can be seen on the picture bellow, illustrating both input and output voltages. The op-amp has very sensitive inputs, thus a very little change between them results saturation in the output.
For our next task we have to build active voltage follower and use it in some effective way. We start with building a simple voltage divider. If we connect a voltmeter in the output we notice that the desired voltage is lower than expected due the voltmeters inner resistance. The whole circuit acts like an actual voltage source. In order to make this ideal we need to compensate via 100% negative feedback. In the previous labs this role in the compensation method was played by a transistor. It was both acting like a comparing and regulating device. Now we'll replace it with an op-amp. The final circuit can be seen on the right.
Now lets shift to our next task-the interesting concept of the gyrator. By definition the gyrator is used to transform a load capacitance into an inductance. This is very useful because the bulky coils can be replaced by smaller assembly containing a capacitor, operational amplifiers or transistors, and resistors. We'll start from scratch with building a simple RC and RL circuits and analyzing the two diagrams representing the change of the voltages and currents of both C and L in the time domain. We notice that the voltage drop on the resistor in the RC circuit represents the voltage drop of the inductive element in the other circuit. After discovering this dependence we'll have to find a way to implement it.
We start of with building a RC circuit. To to copy the voltage drop on the resistor which simulates the inductive element we use another power source as shown on the picture. Next we must find a way to copy the current which flows through the capacitor. For this purpose we focus on the other branch and connect a resistor Rl. The voltage drop on it is equivalent of the drop on the capacitor. The desired current is equal to Vc/Rl. Now the sketch appears again to resemble a familiar thing-balanced bridge. Next step will be to integrate the voltage follower we discussed above into the circuit. Now the combination of the op-amp and the resistor Rl behaves like inductive element.
We didn't have time left for real implementation on the PCB, but instead we came to an interesting discussion about the existence of positive feedback loop in the circuit below. In fact such definitely exist and to be stable the ratio of this feedback must be smaller than 1.

### See Also

Demystifying gyrator circuits

## Lab 5 Building inverting and non-inverting amplifiers with op-amp

After building the voltage follower in the previous lab, now is time to rise the amplifying constant to, lets say 10. By 10 we mean strictly 10, without noticeable bias. For understanding the principle we discussed some real life examples which involve such mechanisms(like the car driver and the accelerator pedal). We came to the fact that in the process of overcoming some kind of disturbance or obstacle the follower becomes an amplifier. So our purpose now is to place such constant obstacle from the output to the input of our comparing device. At first we try to use just a simple resistor but it won't work because there wouldn't be any voltage drop on it due the high input impedance of the op-amp. So we'll use another well known circuit-the voltage divider. So we choose the resistors to be equal and managing ratio of 0,5. In this interesting way we actually built an amplifier by attenuation and pick for output the reaction to that attenuation.
For inventing the inverting amplifier we'll travel back in time in the year when Harold Black stumbled to the same problem. He didn't have any op-amp with differential inputs by his side. So we'll single input device. Following the circuit bellow we try to find a way to sum the voltages of the two batteries. We cannot sum them in serial, but summing them parallel will cause issues, so we will connect two resistors to soften the conflict. This way of connecting them illustrated well the principle of superposition and actually we made a summer with weight factors(electric scales). After we got this summer we'll connect the output to the op-amps inverting input. In case we use op-amp with differential inputs we'll connect the non-inverting input to ground. To make our summer the input and output voltages must be with different poles. So we invert the output voltage and than receive an inverter. To make it amplify we rise the resistance R2 to be bigger than R1 very much like a pair of scales. If the proportion is opposite we'll get an inverting attenuator. We also see that the inverting input of the op-amp appears to be '0' or the so called 'virtual ground', and we see that the potential diagram 'oscillates' around this point. Another benefit of the circuit is that it consumes energy from the power source, not from the input.

### Additional info

Parallel voltage summer and its uses