## 2 Basic Generator Topologies
### Pulse Forming Circuit Topology Based on Transmission Lines  
The other group of pulse forming circuits is based on the charge and discharge of the transmission lines. This is a common type of circuit topology for generation of high-voltage pulses with less than 100 ns duration [39,59]. As illustrated in Figure 8, the pulse forming circuit is composed of coaxial cable, which is used as a conductor of length l and charged through a resistor R to a voltage V. The transmission line discharge and pulse forming are controlled by the switch SW.  
In order to form rectangular pulses the impedance of the load must match the characteristic impedance of the transmission line. However, load matching requirement by design limits the generator's compatibility with dynamic loads since the impedance of the transmission line is dependent on the parameters of the cable.  
The pulse amplitude of a single transmission line generator is limited to the half of the power supply voltage, while two or more lines must be used to match the pulse amplitude voltage to the power supply voltage (Figure 9). This type of configuration is known as a Blumlein transmission line pulse generator. Blumlein pulse generator is one of the most popular designs for the generation of rectangular high-voltage nsPEF pulses [60, 61, 62]. The generator is based on the two transmission lines of identical length l. Both transmission lines are electrically connected at one end to a load and discharged by closing the switch. If the load is mismatched (i.e., the load resistance is larger or smaller than the transmission line impedance), reflections of the voltage step at the load and at the open end of the transmission line will lead to a strain of consecutive decaying pulses [42,63,64]. Hence, to generate rectangular wave pulse without any pulse reflections, the impedance of the load must be twice the impedance of the transmission line. This is not always an easy task to achieve, since the electrical impedance of biological tissue is often unknown, can vary from sample to sample and can be even dynamic during the pulse delivery [38,65].  
Figure 8: The conceptual circuit of a transmission line pulse generator.  
Figure 7: The conceptual circuit of bipolar half-bridge (**a**) and full-bridge (**b**) direct capacitor discharge pulse generator.  
Lastly, the control of pulse duration in a Blumlein circuit is non-flexible. It is defined by the length of the transmission line and the dielectric between the conductors, while the switch-off time determines the rise time of the generated voltage pulse. When the switch is closed, a voltage pulse is generated across the load for the time it takes the voltage step to propagate along the transmission line [42]. The switching element must withstand full high-voltage, which can be a challenge. On the other hand, in case of a spark gap switch, the output amplitudes can reach tens or hundreds of kV and a few kA.  
Lastly, the Blumlein generators are large dimension devices and have high demands for the electrical components. The usual concepts have a relatively short lifetime and operate in low repetition rate, while a rectangular wave pulse is typically delivered with a jitter [38; 63]. However, with the latest modifications, Blumlein generators can now generate also a high-frequency (range of up to few MHz) output pulses with variable duration, amplitude and even polarity [38; 60; 63; 66].