Learn About Two-Transistor SR Latch – How it Works Step-by-Step

A transistor SR latch (also called a two-transistor SR latch) is a basic bistable memory circuit built using only transistors instead of logic gate ICs. It stores one bit of information and shows how positive feedback creates stable logic states in sequential circuits.

Not long ago, we discussed making an SR latch using logic gates instead of specialized SR latch ICs. But what if we do not want to use or have any ICs at all? In that case, a functional SR latch can be built using only transistors and other passive components.

Today, we will find out about that and more. We will learn about the basic design of a transistor-based latch and its step-by-step operating process, and as always, we will also create a simple test circuit at the end to help us better understand this topic.

Back to read: How an SR Latch Works

Before examining the circuit in detail, it helps to briefly review how an SR latch works and how this behavior is implemented using transistors. P.S.: If you already have a rough idea of how an SR latch works, feel free to skip to the next part.

A Brief Overview of an SR Latch

In short, an SR latch is a digital sequential logic with two inputs (Set and Reset) and at least one output (Q). When one of its inputs receives an active signal (“1” logic), its output will change accordingly.

If no active signal is received, it will hold the current output state, also known as the hold state. The function of both inputs when activated is as follows:

• Set (S) → Set output Q to “high” or logical “1.”
• Reset (R) → Reset output Q to “low” or logical “0.”

In most circuits, the “1” logic is usually closer to or at the positive voltage supply, whereas the “0” logic is closer to GND (0V).

Most SR latch constructions also have an inverted output (_Q). This output always has the opposite value of the normal Q output; _Q has the inverted value of Q.

If you want to learn more about this subject, read our in-depth article here. We also discussed using an IC-based SR latch, which is a simpler option if you only need an SR latch in your digital circuit.

Two-Transistor SR Latch Circuit

Now that we know the basics of an SR latch, let’s look at the circuit for a transistor SR latch. It is a simple circuit consisting of two transistors and a couple of resistors, as well as switches.

The two transistors are connected in a cross-coupled manner, where the B (base) of one is connected to the C (collector) of the other. The resistors limit current and prevent direct current flow between VDD and GND.

 

In this circuit, we activate the Set and Reset inputs by connecting (shorting) the B (base) of transistors Q1 and Q2 to the GND. This is where the two push switches (S1 and S2) come in. The switches have the following function when pressed:

• S1 → latch gets the Set command (Q = “1,” _Q = “0”)
• S2 → latch gets the Reset command (Q = “0,” _Q = “1”)

As for why it works this way, we will learn more about it in the step-by-step testing section later.

Now, what about the outputs? This circuit has two outputs, one normal (Q) and one inverted (_Q). When one is OFF (has “0” logic), the other will be ON (has “1” logic).

As labeled in the schematic above, the _Q and Q outputs are drawn from the C (collector) of Q1 and Q2, respectively. Next, let’s see the working table of this latch circuit.

Transistor SR Latch Working Table

The inputs are represented by the switches S1 and S2. “OFF” indicates that the switch is open (does not conduct current). “ON” indicates that the switch is closed (conducts current).

The output columns display the voltage of the outputs (Q and _Q) after receiving specific inputs. QNEXT refers to the next Q value, whereas regular Q refers to the current Q value; the same goes for inverted Q (_Q).

Lastly, the action shows the functions of each input combination. If the table is a little bit too overwhelming, let’s go over it row by row and see what it really means.

• S1 = OFF, S2 = OFF → Voltage at Q and _Q hold the previous values.
• S1 = OFF, S2 = ON → Voltage at Q = 5V, and _Q = 0.1V (known as ON state).
• S1 = ON, S2 = OFF → Voltage at Q = 0.1V, and _Q = 5V (known as OFF state).
• S1 = ON, S2 = ON → Voltage at Q and _Q = 5V simultaneously.

Since this is a latch circuit, when we press and release one of the switches (changing from OFF to ON to OFF), the outputs still hold the voltage values from that input function.

Test Circuit With Simple Load

Now that we know the basic functions of the two-transistor SR latch circuit, let’s learn about its electrical properties. We will hook this circuit up to a 5V power supply, then find the following:

• The voltages at the B (base) of Q1 and Q2 (R and S inputs) in both states.
• The voltages at the C (collector) of Q1 and Q2 (Q and _Q outputs) in both states.
• The direction and order of currents in each state, ON and OFF.

We will also use a simulated load consisting of the resistor R5 (330Ω) and LED1 in the circuit schematic below.

 

During the testing, we will assemble this circuit onto a breadboard. If you are interested in trying out this circuit as well, here are the components you will need:

 

Parts list

Resistors 0.25W 5%

• R1,R4: 820Ω x 2
• R2, R3: 10K x 1
• R5: 330Ω

Semiconductors

• Q1: 2N3904 or C9013 or equivalent
• LED1: 5mm LED

Others

• SW1, SW2  Tactile switch 2 pins
• Breadboard
• Jumper wires
• 5V Power supply

 

Step-by-Step Explanation

When we first power on the circuit, it will start in either the ON or OFF state. For us, the circuit reliably started in the OFF state, even without a load. This may depend on factors such as individual transistor switching speed or resistor tolerance.

Reset Action (OFF State)

Nevertheless, we will begin testing by pressing the S1 switch, which sends a reset command to the circuit (OFF state). The schematic below shows what will happen step-by-step.

Transistors SR Latch When S1 is Pressed (Q = “0”, _Q = “1”)

1. When we press S1 (ON), the bias current (if it exists) at B of Q1 flows through the switch to ground. This is because it is the path of least resistance.
2. In the absence of a bias current, transistor Q1 turns off (stops conducting current from C to E).
3. As a result, the current has no choice but to flow through the higher resistance R2. This current is now a bias current for Q2, which turns it on.
4. After Q2 turns on, the majority, but not all, of the current flows through its C-E to the ground. It should be noted that some bias current continues to flow through B-E of Q2, enough to keep the transistor on.

At the end, there is no current flow over the load because the C-E path through Q2 offers almost no resistance. Thus, the voltage at Q is also almost 0V.

On the other hand, the voltage at _Q (inverted output) is nominally 5V, or logical “1.” If we connect the load to this _Q instead, it would turn on. And because this is a latch circuit, after we release the switch S1, the circuit holds this output state (Q = “0”, _Q = “1”).

Set Action (ON State)

Now, what would happen if we pressed S2 and gave the circuit a set command instead (ON state)? Let’s find out in steps.

 

1. When S2 is pressed (ON), the bias current at B of Q2 immediately flows to the ground through the switch.
2. Without the bias current for Q2, it thus shuts off (stops conducting current via C-E).
3. The higher amount of current is now flowing through the load to the ground, powering it.
4. At the same time, a smaller amount of current passes through R3 to bias Q1 (turn it on). This current is lower than the load because R3 has a higher resistance than R5 (10kΩ vs. 330Ω), allowing less current to flow through.
5. Finally, once Q1 is turned on, current can flow to the ground through its C-E.

This state of the circuit is the mirror opposite of the previous one. But instead of just two paths for current to complete the circuit (reach the GND), there are now three (3, 4, and 5). Also, the output current (5) in this case is relatively higher than that of the logic gate ICs.

One interesting point is the voltage at the output Q. We measure it at 2.8V, which is lower than the _Q in the previous state (5V). This is because of the load, which acts as a bypass for the transistor Q2. If we disconnect the load, the voltage will come back to 5V.

However, if we remove R5 from the load (i.e., there is no resistance between Q and GND), the voltage at Q will be almost 0V (“0” logic), the same as when Q2 is on. The higher the load resistance, the higher the voltage at the output, and vice versa; the ICs also behave like this.

Similar to the previous state, after we release the switch S2, this circuit retains these current output values (Q = “1”, _Q = “0”).

When We Press Both S1 and S2 (Invalid Action)

We are curious about what will happen if we press both S1 and S2 at the same time. So we try it and do some measurements.

When Both Switch is Pressed At the Same Time

You can see that both transistors turn off because their bias currents are shorted to ground. As a result, the current can only reach GND via S1, S2, and the load.

In this case, the voltage at both outputs (Q and _Q) is at “1” logic, or 5V without load, while the voltage at the B (base) of Q1 and Q2 is 0.1V.

Conclusion: Transistors Vs. ICs

The transistor SR latch circuit works very well as a switch-controlled latch. We need to press either S1 or S2 to give it a command. This is because the input is applied by connecting the B of the transistors with the GND.

If we want to control this circuit with a digital signal (i.e., “high” and “low”) like the ICs, we can change both switches with a transistor or comparator instead.

The transistors will function as switches. When we apply “low” logic, it acts as a closed (OFF) switch. But it will act as a closed (NO) switch when we apply “high” logic.

Back to learn: How an SR Latch Works – A Step-by-Step Guide

In conclusion, this transistor-based SR latch circuit is an excellent alternative to the NAND or NOR gate version. It is simpler and easier to understand. Here are the main takeaways for this article:

• We can build a functional SR latch using transistors.
• A simple transistor latch circuit like the one we covered today cannot accept digital input.
• Transistor circuits have less output current restriction than their IC counterparts.

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